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(LifeSiteNews) – In the context of the ongoing COVID-19 pandemic much attention has been given to the personal decision to receive, or not, one of the COVID-19 vaccines. What initially started as strong encouragement to receive experimental vaccines has quickly moved to heavy handed mandates that intimidate, coerce and marginalize large numbers of people around the globe. This three-part series aims to help form one’s conscience by starting with the official teachings of the Catholic Church through the Catechism, applying the Ethical and Religious Directives for Catholic Health Care written by the United States Conference of Catholic Bishops (Part I), a review of recent Vatican issued statements on receiving vaccines in their proper context (Part II) and then concluding with the data of real-world experience (Part III).

Part I: Catechism of the Catholic Church and Ethical & Religious Directives for Catholic Health Care

In mid-2021 multiple statements and guidance on COVID-19 vaccination were issued by various dioceses across the United States, with some supporting religious exemptions to vaccine mandates for the COVID-19 vaccine, and others arguing against religious exemptions.1 In order for one to have a well-formed, sure conscience on whether to take the COVID-19 vaccine, it is important to assess the latest and best available data regarding a rapidly evolving pandemic.

Given the amount of constantly changing medical and scientific data, as well as articles from the Catholic perspective on the topic of vaccine mandates from around the globe, it is important to begin with the most recent and relevant events and data that have brought us to this moment in time. Each of the three currently available vaccines in the United States has some degree of connection to abortion line cells. The vaccine with the strongest connection to abortion cells is the Johnson and Johnson vaccine that used abortion line cells in the design, production, and testing phases of the vaccine development. In comparison, both messenger RNA (mRNA) vaccines, Moderna and Pfizer-BioNTech, used abortion cell lines during the testing phase of vaccine development.

With the varying degrees of connection to abortion of the currently available vaccines established and well covered in other sources, this work will focus on the relevant data that holistically informs one’s conscience on the decision of whether to receive an experimental vaccine. It is important to note that when the Church declares a medical treatment illicit due to its connection to abortion cells, such a judgment is made by evaluating the relevant real-world facts and data, and applying the conscience to make such a determination. Therefore, it would be incorrect and contradictory to state that one cannot discern declining medical products or treatment for interconnected relevant, well-formed reasons of conscience. Furthermore, just because something is declared morally permissible does not de facto make it morally obligatory. Each medical treatment must be evaluated on an individual basis considering all potential benefits, risks and connection to abortion.

Elements of conscience formation in relation to medical products

A well-formed conscience begins with an examination of the elements necessary for conscience formation pertaining to medical products. An appropriate starting point is the Catechism of the Catholic Church, specifically Article 6 regarding Moral Conscience.2 The Catechism begins by teaching that the Moral Conscience is man’s most secret core and sanctuary, where he is alone with God (Catechism, n. 1776). Man has the right to act in conscience and in freedom and must not be prevented from acting according to his conscience or forced to act contrary to his conscience (Catechism, n. 1782). The Conscience must be informed and moral judgment enlightened (Catechism, n. 1783); education of the conscience is a lifelong task that guarantees freedom (Catechism, n. 1784).

Man must always seek what is right and good (Catechism, n. 1787) by interpreting the data of experience and signs of the times (Catechism, n. 1788). Charity must always respect one’s neighbor and their conscience because to violate another’s conscience is to sin against Christ (Catechism, n. 1789). One must obey the certain judgment of their conscience because to act contrary is to condemn oneself (Catechism, n. 1790).

Each of the primary elements necessary for a well-formed conscience in the context of medical products can be discerned by applying the Ethical and Religious Directives (ERDs) for Catholic Health Care Services developed by the United States Conference of Catholic Bishops (USCCB). The purpose of the Directives is to provide authoritative guidance on moral issues in modern health care. It begins by discussing how new medical discoveries can be either a time of great advancement, or lead to policies and actions contrary to the true dignity of the human person. The ministry of Catholic health care is committed to promoting and defending the dignity and sacredness of every human life from the moment of conception until natural death. Catholic health ministry does not offend the rights of individual conscience and must avoid manipulation, intimidation or condescension.

An essential element of medical treatment and procedures is the free and informed consent of the person. In order to be able to give free and informed consent the person must receive all reasonable information about the essential nature of the proposed treatment including its benefits (in the case of vaccines effectiveness), risks, side-effects, consequences, and any reasonable and morally legitimate alternatives, including no treatment at all.

The Directives go on to state that no one should be subject to medical or genetic experimentation (which any medical product with unknown long-term consequences should be considered, and all currently available US vaccines are still considered) without first having given free and informed consent. Furthermore, “no person should be obliged to submit to a health care procedure that the person has judged, with a free and informed conscience, not to provide a reasonable hope of benefit without imposing excessive risks and burdens on the patient.”

One last point raised in the Directives is that any procedure or product that may cause harm or undesirable side-effects can be justified only by a proportionate benefit to the person. Having identified the necessary and relevant elements to forming conscience related to medical products, it is prudent to review recent Vatican guidance on receiving vaccines in their proper context.

Part II: Vatican statements on receiving vaccines in context

When developing broad policy with far-reaching implications, it is important to consider the context and specifics of the material used to justify such policy. As a starting point, many articles and resources circulating in the Catholic sphere that support vaccine mandates often begin by referencing the Pontifical Academy for Life’s (PAV) 2017 document, Note on Italian Vaccine Issue. This guidance was developed in the context of a declining measles and rubella vaccination rate in the Italian population.

Here the document states, “The moral obligation to guarantee the vaccination coverage necessary for the safety of others is not less urgent, especially the safety of more vulnerable subjects such as pregnant women and those affected by immunodeficiency who cannot be vaccinated against these diseases.”

In the context of the situation leading to the PAV’s position, several important points that differentiate measles from COVID-19 indicate that the strong presumption in favor of requiring the former cannot easily be generalized to the latter.

First, measles is significantly more contagious and deadly than even the Delta or Omicron variants of COVID-19, resulting in an estimated six million deaths a year prior to the measles vaccine. Second, the measles vaccine was originally licensed in 1971, and we have decades of data showing that the virus is stable and that the vaccine is 97 percent effective. This preponderance of evidence does not exist for COVID vaccines. Finally, the 2017 document states one should get vaccinated for measles to protect those who cannot be immunized, such as pregnant women and those with immunodeficiency. However, for COVID-19 the current recommendation by the US Centers for Disease Control and Prevention (CDC) is that pregnant women and, after consultation with their doctors, some who are immunocompromised can receive one of the COVID-19 vaccines.

The next, and arguably more authoritative, document issued by the Church on the subject of vaccines came from the Congregation for the Doctrine of the Faith in its Note on the Morality of Using Some Anti-Covid-19 Vaccines, dated December 21, 2020. The document states:

Practical reason makes evident that vaccination is not, as a rule, a moral obligation and that, therefore, it must be voluntary. In any case, from the ethical point of view, the morality of vaccination depends not only on the duty to protect one’s own health, but also on the duty to pursue the common good. In the absence of other means to stop or even prevent the epidemic, the common good may recommend vaccination, especially to protect the weakest and most exposed. Those who, however, for reasons of conscience, refuse vaccines produced with cell lines from aborted fetuses, must do their utmost to avoid, by other prophylactic means and appropriate behavior, becoming vehicles for the transmission of the infectious agent. In particular, they must avoid any risk to the health of those who cannot be vaccinated for medical or other reasons, and who are the most vulnerable. (Bold emphasis added)

The most pertinent point is the change in language from vaccines being a moral obligation in 2017 to a moral responsibility in 2020. This is important within the context of Emergency Use Authorization (EUA) at the time the CDF document was written, where both the real-world effectiveness (compared with clinical trial efficacy) of the available vaccines and the potential long-term side effects of vaccines using new mRNA technology are unknown.

In fact, recent real-world data on the Delta variant suggest the vaccines are less than 50 percent effective; completely ineffective against Omicron with an increased incidence of disease with each additional shot; do not prevent viral transmission; may increase the probability of variants; and have a much higher associated risk and mortality compared with other well-established, safe, and effective vaccines. These data invalidate the moral-obligation argument and make the moral permissibility of vaccines with ties to abortion cell lines less acceptable in the case of COVID vaccines than for the MMR vaccine.

Eight days after the publication of the CDF document, the PAV and the Vatican COVID-19 Commission issued a joint paper, Vaccine for All. 20 Points for a Fairer and Healthier World, that reflected the corrected language of “the moral responsibilityof undergoing vaccination.”

This change in language from the 2017 PAV document on the MMR vaccines to the 2020 Documents on COVID-19 vaccines is significant because it brings the 2020 documents into alignment with the Ethical and Religious Directives and Catechism n. 1782, that “Man has the right to act in conscience and in freedom so as personally to make moral decisions. ‘He must not be forced to act contrary to his conscience. Nor must he be prevented from acting according to his conscience, especially in religious matters.’”

Part III: The data of real-world experience

Clinical efficacy vs real-world effectiveness: disease prevention and transmission

Having established proper context and relevant timelines, the documents can then be properly applied to the specific context of COVID-19 vaccines. The release of the December 2020 documents by the CDF and the PAV coincided with the EUA approval of the Pfizer and Moderna mRNA vaccines in the United States. At the conclusion of the clinical trials in December 2020, primary efficacy was 95 percent for Pfizer and 94.5 percent for Moderna.

In research, a primary endpoint is “the main result that is measured at the end of a study to see if a given treatment worked.” For COVID-19 vaccine efficacy trails, this was defined by the FDA as laboratory-confirmed infection with either COVID-19 or SARS-CoV-2. Widely deployed vaccines had to be at least 50 percent efficacious at preventing infection, and the lower bound of the confidence interval had to be least 30 percent.

In the same document, the FDA identified severe disease including death, of the vaccine recipient, as a secondary endpoint. The safety follow up for both vaccines was noted in the respective EUA applications as 24 months after the second dose, which still has not been completed. The FDA’s press release on the subject conceded that “information is not yet available about potential long-term health outcomes.” other new medical treatments using related technologies, such as gene therapy, the FDA recommends a fifteen-year long-term follow up (LTFU).

Preceding the EUA approvals, as early as the summer of 2020, the segments of the population that were at elevated risk for severe COVID-19, including older age groups and those with one or more underlying medical conditions, were already known.

Considering all the available information at the time of the EUA approval, one might have argued to either receive or decline the vaccines. If one was in an older demographic with one or more underlying medical conditions the reported benefits the COVID-19 vaccines at the time could outweigh the risks associated with an experimental vaccine and a COVID-19 infection. Conversely, if one was younger, generally healthy and did not have an underlying medical condition, the risks associated with an experimental vaccine, including unknown long-term consequences may have offered little to no benefit to the recipient. It is then imperative for those who would experience questionable benefit from a COVID-19 vaccine to monitor vaccine efficacy at preventing infection over time and the associated personal risk through the adverse events reported.

As the vaccine was rolled out worldwide, early reports indicated the real-world effectiveness was promising and aligned with the clinical results. However, as time passed and variants developed and spread, real-world data showed waning primary effectiveness.

On August 21, 2021, the Mayo clinic reported findings that indicate a reduction in vaccine effectiveness against infection over time. Compared with prior months, in July Moderna decreased in effectiveness from 86 percent to 76 percent, and Pfizer decreased from 76 percent to 42 percent. While public health authorities and pharmaceutical companies asserted that both mRNA vaccines achieved high effectiveness for secondary endpoints (severe disease, hospitalization, and death) at the time, the primary endpoint effectiveness values for the Pfizer vaccine dropped below the EUA target of 50 percent. Subsequent studies published in Science, The Lancet and The New England Journal of Medicine all had similar findings.

At the time of publication only the Moderna vaccine retained a reported primary efficacy of preventing disease above 50 percent, however projections for Moderna in Cohn et al. anticipated it also decreased to less than 50 percent primary efficacy. This is an important finding in that any COVID-19 vaccine that has an effectiveness in preventing disease less than 50 percent would not meet the FDA’s criteria for a widely distributed vaccine. The negative efficacy predicted by Nordstrom et al. in The Lancet appears to be realized in the Public Health Scotland data with the circulation of the Omicron variant showing those who have received two shots of the vaccine with an increased likelihood of disease, hospitalization and death.

A confounding issue with the effectiveness discussion is that as of May 1, 2021, the CDC stopped tracking breakthrough infections except for hospitalizations and deaths, thus making primary effectiveness determinations difficult at best. With significantly waning effectiveness of the COVID-19 vaccines in preventing infection over time, a person in an elevated risk group may then have asserted that the potential benefits of the vaccine outweighed the known reported risks and elected to receive the vaccine like in the case of the Delta variant.

On the other hand, significant decrease in effectiveness in preventing the Delta variant rejected the position that one in a low risk or healthy population should receive the COVID-19 vaccines for the common good. Moreover, the negative effectiveness against Omicron, along with the medical risks, would have justified rejecting the vaccine regardless of a patient’s risk status. In this case, an individual should not have felt spiritually, medically or publicly pressured to accept an experimental vaccine.

The subsequent argument often proposed from the practical and Catholic moral position is that even if the vaccine is not as effective in preventing disease as originally thought, one should still get the vaccine to prevent spreading infection to the most vulnerable for the common good. The primary assumption in such a position is that even if one has a breakthrough infection, he or she is not, in turn, able to spread the disease to others.

One of the first reported most compelling pieces of evidence against such a position was reported in a study on July 31, 2021 by the University of Wisconsin-Madison that showed that the viral load in the unvaccinated and vaccinated breakthrough cases were essentially the same. Specifically, the findings indicate that high viral loads are present in infected individuals despite vaccination and that high viral loads are consistent with the potential to transmit SARS-CoV-2 regardless of the individual’s vaccination status.

Community-level research provides additional evidence that vaccinated persons can transmit the disease. In a July 2021 outbreak in Barnstable County, Massachusetts, 74% of the reported infections occurred in those who had been fully vaccinated, which was one of the factors that prompted the CDC to change its mask guidance for those who have been fully vaccinated. Additionally, the territory of Gibraltar, which had a reported vaccination rate of 100 percent as of April 30, 2021, saw a wave of COVID infections in July, 2021 with the spread of the Delta variant.

In the United States, states that were leading in vaccination rates at the onset of Delta variant, such as Vermont (67 percent as of August 12, 2021), also saw a wave of infection of COVID-19 that started late July to early August, 2021 with the Delta variant. On July 26, 2021, Israel, having a fully vaccinated rate of 57 percent, reported that 60 percent of COVID-19 hospitalizations occurred among people who had been vaccinated.

Similarly, Singapore, having a fully vaccinated rate of 75 percent in July of 2021, reported that 75 percent of COVID-19 cases were among those who had been vaccinated at least partially. Such trends continued and even increased with the spread of the Omicron variant even with the rollout of booster campaigns in various parts of the world.

Given both the individual- and community-spread evidence in all of the previous instances, there lacks a compelling case that the vaccine protects others and should be received by all for the common good. In fact, an article in The Lancet Infectious Diseases discussing the vaccines effect on transmissibility “highlights that the vaccine effect on reducing transmission is minimal in the context of delta variant circulation.” The recent waves of COVID-19 infections with the Delta and Omicron variants in highly vaccinated populations highlight the need to further investigate the possibility of antibody-dependent enhancement (ADE), or the ability of suboptimal antibodies to assist the virus, that was originally raised as a potential concern for COVID-19 vaccines and the occurrence of antibody-resistant SARS-CoV-2 variants in vaccine breakthrough cases.

A final consideration related to disease prevention and transmission in relation to the common good argument is the number of hospitalizations and deaths believed to have been prevented by vaccination. Such an argument is highly speculative and the resulting estimates vary widely with the effectiveness values used, which are constantly changing, and makes the underlying assumption that the vaccine recipient was challenged by the virus which given the large number of worldwide lockdowns is not necessarily valid.

Therapeutic benefits and risk assessment

A balanced and holistic approach to a well-formed conscience also includes an individual risk assessment that weighs the therapeutic benefits against the risks. In the case of an experimental vaccine that lacks decades of data or knowledge of long-term effects, one source that can be used for individual risk assessment is the Vaccine Adverse Effects Reporting System (VAERS). While the limitations and criticisms of the VAERS database are known – including the inability to establish causality, the ability for anyone to submit claims, and the fact that not all claims are verified – the database has been used to assess the safety of new vaccines.3

To account for the limitations of the VAERS database, while still using the data it collects to perform a relative risk assessment, comparison to another widely administered vaccine in the VAERS database would be subject to the same limitations. A natural candidate drawn from the Church’s own comparison, the MMR vaccine, having decades of reporting data, is used. When the VAERS database is queried for deaths, hospitalizations, life threatening events and permanent disabilities for all locations, the following results were obtained on 8/12/2021:

Table 1: Raw number of AE reports for COVID-19 and MMR Vaccines

Category COVID-19 Vaccine (CV) MMR Vaccine (MV)
Deaths 12,791 432
Hospitalizations 51,242 6,934
Life Threatening Events 13,139 1,326
Permanent Disability 16,044 1,822

To put these results in perspective, given the COVID-19 vaccines (CV) were available for less than a year at the time the data was originally pulled and the MMR vaccine (MV) has been available for 50 years with 31 years of VAERS reporting data available, the data in Table 1 is normalized by the number of years of reporting data (8 out of 12 months or 2/3 of a year for CV and 31 years for MV) to obtain the adverse event (AE) rate. Table 2 shows the number of AEs reported on a per year basis. Taking the CV AE rate and dividing by the MV AE rate, one can assess the number of times an adverse event is more likely to occur for the COVID-19 vaccine as compared to the MMR vaccine, based on reporting data to assess relative risk.

Table 2: Adverse Event Rate and Relative Risk

Category COVID-19 Vaccine/year MMR Vaccine/year Factor (CV/MV)
Deaths 19,187 5 1377
Hospitalizations 76,863 80 344
Life Threatening Events 19,709 21 461
Permanent Disability 24,066 25 409

While the results in the last column of Table 2 may be alarming to some, that is not the intent. Given that there may be overlap between life threatening events, hospitalizations and deaths, the data in Table 2 can be reduced to permanent disability and deaths since the two are unlikely to overlap. Furthermore, the data in Table 2 does not account for the number of people having received each vaccine so it is appropriate to further normalize the data on a per 100,000 person basis, or incidence, as many population based statistics are reported during the pandemic.

An additional way to improve the analysis is to limit the data to that reported in 2021 for both vaccines since awareness and ability to report would be similar during the same time period.

One of the most important results from Table 2, given the original analysis was performed in August, is the number of adverse events for the COVID-19 vaccines it predicted. As of December 10, 2021, which is almost a year exactly since the COVID-19 vaccines started being administered, the reported adverse events for deaths and permanent disability were 21,632 and 34,970 respectively which exceeds the number of events in a year Table 2 predicted in August 2021.

Continuing with an incidence analysis, approximately 3.6 million babies were born in 2020 and 90.8 percent typically receive the MMR vaccine. It is assumed for the analysis that the number of births in 2021 is the same as 2020 and the percentage who receive the MMR vaccine is then applied to that number. The number of people reported to be fully vaccinated with the COVID-19 vaccine is approximately 203.2 million people in the United States.

Data is then extracted from the VAERS database limited to the adverse events of death and permanent disability, only in the United States and only for those events reported in 2021. The adverse events of death and permanent disability for the COVID-19 and MMR vaccines are then normalized per 100,000 people having received the vaccines and reported in Tables 3 and 4 respectively.

Table 3: Incidence of reported Deaths for COVID and MMR vaccines

Vaccine Vaccinated Adverse Events Incidence per 100,000
COVID 203,200,000 10,670 5.25
MMR 3,268,800 5 0.15


Table 4: Incidence of reported Permanent Disability for COVID and MMR vaccines

Vaccine Vaccinated Adverse Events Incidence per 100,000
COVID 203,200,000 12,139 5.97
MMR 3,268,800 27 0.83

As can be seen from the incidence values for both vaccines there is a greater than 34 fold increase in reported deaths for the COVID-19 vaccines compared to the MMR vaccine and a more than 7 fold increase in the incidence of permanent disability. The increase in incidence for the adverse events of death and permanent disability for the COVID-19 vaccines is significant.

Similarly, a recent study looked at age-stratified all-cause mortality, or deaths in a population regardless of cause, investigating Vaccine-induced Fatality Rates (VFR) using publicly available data from multiple parts of the world. The results showed that all-cause mortality increases coincided with prior vaccination timelines for that same age group that were not seen during prior COVID waves.

Such findings show a correlation between the COVID-19 vaccines and an increase in all-cause mortality. Risk and safety are both relative terms, as there is no such thing as “risk free” or “absolute safety.” Everything in life has inherent risk, and safety, in a practical definition, is the absence of unreasonable risk. What is considered an acceptable level of risk by some may not be by others. It is only through a well-formed conscience, with the assistance of the data of experience and the guidance and wisdom of the Holy Spirit, that one can discern what is reasonable risk for oneself.

It is reasonable for one to look at the results presented in Tables 2 through 4 and conclude, with a well-formed and sure conscience, that based on the reported severe adverse events for the experimental vaccines, personally mitigating risk factors for severe disease and death, low primary effectiveness of the COVID-19 vaccines, the ability of those who have been fully vaccinated to infect others, and the unknown long-term potential side effects of the vaccines, that the therapeutic benefits do not outweigh the elevated risk associated with the COVID-19 vaccines.

In the case of children, with the emerging evidence on vaccine induced myocarditis, where there is no benefit and only risk, to compel vaccination is not consistent with the common good and Catholic teaching. Since the emergence of the Omicron variant, the vaccine has shown negative effectiveness in which the vaccine is correlated with higher case rates as shown previously.

Thus, an individual would not even need to struggle with the medical or moral decision on whether to take the vaccine regardless of their risk status. Over the course of the last year with the introduction of COVID-19 vaccines, the data of real-world experience has seen a significant decrease in efficacy of the COVID-19 vaccines, a significant increase in both number and severity of reported adverse events post vaccination, and an increasing body of evidence of effective early treatments that obviate the need for vaccination.

Well-formed and sure conscience discernment

All of the above data did not exist when the Congregation for the Doctrine of the Faith and the Pontifical Academy for Life documents were released in December 2020. The evidence offered here provides the most relevant information to form a holistic, well-formed, and sure conscience to inform the decision to vaccinate or not.

Therefore, it is reasonable, given the data of real-world experience of the pandemic and the vaccines’ inability to reduce community transmission, that one could come to a sure conviction, and thus a sincere religious belief before God, that the elevated risks associated with the COVID-19 vaccines outweigh temporary potential therapeutic benefits for most if not all groups. In light of this, instances of the faithful being denied religious exemptions and being faced with either violating their conscience or losing their jobs, including those that work in the Vatican, applies significant undue pressure to violate one’s conscience and, as the Catechism states, “A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself” and is, therefore, a grave matter of religious principle and a direct denial of the dignity of the human person (Catechism, n. 1790).

Stating that for one to get the COVID-19 vaccine is an act of love is to dismiss the relevant data of real-world experience that helps form one’s conscience; and to force one to get a COVID-19 vaccine against their well-formed conscience is contrary to love. Love, as the Catholic Church has long professed and Christ himself exemplified, must be freely given.


[1] Joe Bukuras, “Chicago, Philly Archdioceses Tell Priests Not to Provide Religious Exemption from COVID Vaccines,” Catholic News Agency, August 19, 2021; Archdiocese of Los Angeles, “Statement on Religious Exemption for Vaccination Against COVID-19,” August 16, 2021; Matt Hadro, “NY Archdiocese Warns Priests Not to Grant Religious Vaccine Exemptions,” Catholic News Agency, August 2, 2021, and “Archbishop Etienne: No Religious Exemption to Vaccine Mandates,” editorial, Northwest Catholic, Aug 19, 2021.

[2] Catechism of the Catholic Church, 2nd ed. (Washington, DC: US Conference of Catholic Bishops/Libreria Editrice Vaticana, 2016 update), n. 1781, citing Vatican Council II, Dignitatis humanae (December 7, 1965), n. 3. All subsequent citations appear in the text.

[3] For the limitation of VAERS data, see VAERS, “Guide to Interpreting VAERS Data,” accessed November 19, 2021, https://vaers.hhs.gov/data/dataguide.html. See also, Elaine R. Miller et al., “Surveillance for Adverse Events Following Immunization Using the Vaccine Adverse Event Reporting System (VAERS),” in Manual for the Surveillance of Vaccine Preventable Diseases, ed. Sandra W. Roush et al., last reviewed February 23, 2021, https://www.cdc.gov/vaccines/pubs/surv-manual/chpt21-surv-adverse-events.html. “Clinic staff at the local level are responsible for completing a VAERS report when an AE [adverse event] is suspected or occurs following immunization. From 2012 through 2016, vaccine providers submitted 32% of U.S. VAERS reports; vaccine manufacturers submitted 41%; patients or parents submitted 13%; and 14% came from other or unknown sources.” In addition, VAERS is subject to varying degrees of underreporting.