Vaccine Hesitancy & the Science Behind Common Concerns

By Amichai Perlman, PhD, PharmD
Medically reviewed checkmarkMedically reviewed
April 29, 2021

In a recent poll, we asked how people felt about the Johnson & Johnson vaccine, in light of the FDA & CDC’s decision to pause use earlier this month.

With the pause now lifted, the most interesting learnings may be from those who say they’re not getting any form of the COVID-19 vaccine—K’s vaccination expert, Dr. Amichai Perlman breaks down the science behind their concerns.

In recent months, over 140 million people in the US have received one of the FDA-approved COVID-19 vaccines. The vaccines are now becoming accessible to all adults, and the prospect of getting protection from COVID-19 and gradually returning to normalcy is a source of joy for many.

However, these feelings are sometimes accompanied by apprehension and anxiety over receiving a new medical treatment. All of these feelings and concerns are valid and perfectly reasonable. While very effective at preventing COVID-19 and its severe complications, the vaccines also have some uncomfortable side effects, and like all medical treatments, are associated with very rare but severe adverse reactions.

At K we strive to empower our community with fact-based information regarding medical conditions and treatments. We believe this is key to improving our health and well-being. As there is a lot of misinformation circulating regarding the vaccines, we would like to provide accurate and reliable information regarding common concerns related to the COVID-19 vaccines.

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The vaccines were made too quickly & not tested thoroughly enough.

The COVID-19 vaccines approved by the FDA have undergone extensive testing of their safety and efficacy. The technologies being used in each of the vaccines has been developed over decades of research, and has been tested in previous years for other diseases. Each of the vaccines developed for COVID-19 was studied and tested in a series of studies, starting with cell tissues and animals, and followed by a series of trials in humans which culminated in large clinical trials with tens of thousands of participants. These trials provided sufficient data to conclude that the benefits of the vaccines outweigh known and potential risks, resulting in their approval by many medical agencies around the globe. Following authorization, the vaccines have been administered to hundreds of millions of people worldwide, with careful and continued monitoring of side effects.

The clinical trials and research prior to their authorization proved these vaccines are very effective at preventing COVID-19 and ruled out the possibility they cause severe side effects in even a fraction of a percent of people. The continued monitoring of the vaccines following approval allowed for the detection of even more rare side effects, such as severe allergic reactions with the Pfizer vaccine, estimated to occur in 1 in 200,000 people. This information has led to the development of policies and medical guidance to further minimize and address even these very small risks.

I already have immunity.

People who have already had COVID-19 are at lower risk of being reinfected, however they are not completely immune and they can get reinfected and even experience severe disease. Recent studies have reported lower rates of protection among people who had recovered after having had COVID-19, than the protection observed in most of the vaccine studies. This may be especially true among vulnerable populations, such as older adults. A recent study from Denmark observed only 47% protection from recurrent infection among those over age 65. Current research also shows that following vaccination people have higher levels of antibodies than people who have had COVID-19, and higher likelihood of antibodies which neutralize the newer “variants”. Overall, the data on vaccine efficacy and duration of immunity is generally more robust than that of previous infection.

The vaccines have also been tested and used in people with previous infection, and no differences in safety were observed compared to those who were never infected. Accordingly, the CDC recommends vaccination irrespective of previous COVID-19 infection. The CDC does note, however, that the risk of reinfection is low in the months after initial infection but may increase with time due to waning immunity.

Interestingly, there are some reports and surveys suggesting a reduction in post-COVID-19 symptoms after getting vaccinated, however these reports are very preliminary and require further research to be validated. So bottom line – people who have had COVID-19 have a low risk of reinfection, however vaccination is the surest way to ensure maximal protection from COVID-19.

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I’m already at a high risk for clots, I don’t want to increase this risk.

It is worth starting by noting that the risk of all types of blood clots is substantially increased among those who contract COVID-19, and is orders of magnitude greater than the risk of blood clots for some of the COVID-19 vaccines. Consequently, people with multiple health issues and a high risk of clots stand to benefit most from being vaccinated.

The FDA and CDC have recently paused the use of the Johnson & Johnson COVID-19 vaccine to review reports it is linked to a rare but severe type of blood clot (cerebral venous sinus thrombosis) which appears together with low blood platelets (thrombocytopenia). The review identified 15 cases of this condition among ~7 million recipients of the J&J vaccine. These cases all occurred in women aged 18-59. While very rare, the condition was judged to be plausibly related to the vaccine as the cases had clear clinical and laboratory features, and a similar side effect had been recently identified for the AstraZeneca (AZ) vaccine which is being used in Europe and Britain.

Both the J&J and the AZ vaccines are based on similar technology. They both use a benign type of virus called adenovirus, modified to carry DNA encoding a single protein related to the coronavirus. The scientists who identified this side effect believe that in very rare cases, injecting the adenovirus causes the development of antibodies which activate blood platelets and cause them to create rare types of blood clots.

The FDA and CDC concluded incidences of clots are extremely rare.

And the risk of death and other serious outcomes from COVID-19, including blood clots, is far greater. Accordingly, the benefits of the vaccine were judged to outweigh all known or potential risks of this vaccine. The pause on the vaccine was therefore lifted, and the American Society of Hematology issued guidance for clinicians to help identify and treat the rare cases where this complication occurs. According to this guidance, standard risk factors for blood clots may not be related to increased risk for this complication, as there is no information to date on any increased risk for this complication in patients with blood diseases and/or pre-existing risk factors for clotting or autoimmunity.

The Pfizer and Moderna vaccines are based on mRNA encapsulated in tiny soap-like bubbles and do not contain an adenovirus. Over 200 million doses of these vaccines have been given in the US alone – and to date they have not been linked to an increase in this, or any other, type of blood clots.

Additional information addressing other questions and concerns on the COVID-19 vaccine is available here.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Amichai Perlman, PhD, PharmD

Dr. Perlman is a clinical pharmacist and pharmacoepidemiologist, with over 10 years of experience advising patients and clinicians on medication use, personalization, and safety. He has extensively published peer-reviewed research addressing medication safety.

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