COVID-19 Vaccine Safety

February 26, 2021

This article was medically reviewed by Dr. Ronald Nahass.

During the COVID-19 vaccine rollout, so many people across the world have stepped up to the plate and done their part in trying to end the pandemic through a global immunization effort. However, some people are still hesitant about receiving the vaccine due to concerns surrounding COVID-19 vaccine safety.

So are the COVID-19 vaccines safe? The answer is, yes. And while vaccine hesitation is understandable – given how new the vaccinations are – it is detrimental to the ongoing effort to end the pandemic. ID Care is the largest network of infectious disease experts on the East Coast and has been responsible for treating over 20% of the hospitalized COVID-19 cases in New Jersey. Because of this, the infectious disease physicians at ID Care understand how important it is for people to get vaccinated in order to protect and save as many lives as possible.

That being said, every individual deserves to feel comfortable when getting vaccinated. In order to make that possible, ID Care President and Co-Founder, Dr. Ronald Nahass, will do his best to answer some of the most pressing questions surrounding vaccine safety, including how the vaccines were evaluated through clinical trials, the safety standards of current vaccine technology, and why some people—such as pregnant women and those with compromised immune systems – shouldn’t fear the vaccine.

How does the Coronavirus vaccine work?

Let’s start at the beginning. The current immunizations available in the US – the Moderna and Pfizer COVID-19 vaccines – are mRNA vaccines designed to create an immune response that tricks the body into thinking it has been infected by the virus without actually getting the recipient sick. When this happens, the body naturally creates antibodies that protect the recipient from any future infection. It’s important to remember, when it comes to safety concerns, that getting the COVID-19 vaccine will not make a person ill – it will only improve a person’s chances of staying well and not getting infected with the coronavirus.

Do the COVID-19 vaccines prevent someone from getting infected and from spreading the disease?

The current vaccines available in the US will absolutely – with about 95% efficacy – protect a person from becoming infected with COVID-19. However, it is still not fully understood whether or not the vaccines prevent a person from transmitting the virus to others. There is the potential, after receiving the vaccine, of harboring the virus in the nose or mouth, and then transmitting the virus. This is why it is still recommended to wear a mask post immunization until enough people get vaccinated.

During clinical trials, how was the effectiveness of the COVID-19 vaccine measured?

There are two principal ways to measure the effectiveness of a vaccine. First, scientists and researchers look at a test tube perspective where the level of antibody is measured in the blood of a patient who has received the vaccine. This number is useful, and can be directly correlated to the protective effectiveness of the vaccine.

However, the second way to measure the effectiveness of a vaccine is far more important: how well the vaccine works in real life. That is, if a vaccinated person gets exposed to the virus, what is the likelihood that that person will become infected as opposed to a person who has not been vaccinated. And all coronavirus vaccines have been tested in this way. Trial participants were given either the vaccine or a placebo, and researchers compared the rate of infection in the people who received the vaccine compared to the rate of infection in the people who received the placebo. The difference between the two is how the level of protection was measured. In the case of the vaccines that are currently available in the United States – the Moderna and Pfizer vaccines – the level of protection was measured at about 95% for both. This is a truly remarkable achievement in protection for any vaccine, let alone a vaccine for a new disease that has only been around for about a year.

How were the side effects of the COVID-19 vaccines evaluated?

Similar to the way the effectiveness of the vaccines were tested, the rate of side effects were compared in people who had received the vaccine compared to those who had received a placebo. In the case of both the Moderna and the Pfizer vaccines, there were about 80,000 individuals involved in the studies. That means 40,000 patients received the vaccine, while 40,000 received a placebo. And this number gave researchers a great opportunity to evaluate side effects in a large number of individuals.

The findings of the clinical trials revealed that the most common side effects were similar to those of other vaccines, such as those for flu or tetanus. About 80% of patients complained of a sore arm about a day or two after receiving the vaccination, which is to be expected. Now mind you, about 20% of patients who received the placebo also complained of sore arms, which means there’s about a 60% chance that a sore arm is a side effect of the COVID-19 vaccination. Outside of sore arms, other side effects that were observed were fatigue, headaches, body aches, and muscle soreness. And again, these are some of the most common side effects of any vaccine – which is very promising when it comes to concerns about the safety of the COVID-19 vaccination.

So far, none of the reactions to the vaccine have been severe or life-threatening. However, as the vaccine is being rolled out to a larger number of people, some things are being observed that were not seen in the clinical trials. Several individuals have suffered from an anaphylactic or allergic-type of reaction. But the number of these occurrences have been very small – about one in ninety thousand – and those who did have such a reaction were treated and sent home without any further medical attention needed.

Who was included in the clinical trials for the Coronavirus vaccines?

The participants within the clinical trials were a major reason as to why the vaccines were developed so quickly. However, they were also the reason why the COVID-19 vaccines are so safe and effective for nearly every individual. The participants in the clinical trials were an extremely diverse group, and not just ethnically or racially, but also when it comes to age – ranging across nearly every decade. The diversity also included people with comorbid conditions (i.e., obesity, heart disease, lung disease, etc.) which was very important because COVID-19 has been shown to affect people with certain illnesses or certain underlying health concerns more severely than others. The participants involved reflected a great distribution across individuals that were important to understand their responses to the vaccines. So people of all ages, ethnicity, and health should feel comfortable about the safety and effectiveness of the vaccines. That being said, is the COVID-19 vaccine safe for children? It is of note, no children under the age of sixteen were enrolled in the trials. A trial for children is currently being looked at, and is set to start over the next month or two. So, children younger than sixteen still need to wait to get vaccinated until more information is gathered and evaluated.

What is an mRNA vaccine?

This is a very common question, filled with a bit of unnecessary confusion. The COVID-19 vaccines from Pfizer and Moderna are both mRNA vaccines. This means that they use mRNA to create an immune response within the body. For some, however, when they hear mRNA they think of DNA, and there is a notion that the vaccine will impact or change a person’s genes. This has seemed to become a major concern when it comes to COVID-19 vaccine safety. However, it simply isn’t true. DNA and mRNA are very different.

DNA, or deoxyribonucleic acid, is a person’s genetic code. It lives in the part of the cell called the nucleus. mRNA lives in the part of the cell called the cytoplasm. The relationship between the two is simple: DNA codes for mRNA to tell cells in the human body what to do. For example, in the pancreas, mRNA is coded by DNA to tell the body to make insulin so that blood sugars are controlled and diabetes is prevented. Simply put, mRNA is the messenger.

In fact, the m in mRNA stands for messenger, while the RNA stands for ribonucleic acid. mRNA is not DNA, it is RNA, and messenger RNA is part of the communication system within the human body. More importantly, mRNA is part of the communication system within almost every living organism, including COVID-19.

In the case of the coronavirus vaccines, scientists have figured out a way to take a small portion of mRNA from the SARS-CoV-2 virus – the virus that causes COVID-19 – and package it in a fat globule. This fat globule is then injected into a patient’s arm, and this small portion of mRNA tricks the body into thinking it is sick which causes the immune system to react and produce antibodies that can help protect that patient against the virus. Even better, the portion of mRNA that is injected into the body only codes for a small piece of the virus, and as a result, the body does not have the ability to actually make the virus and get the patient sick.

While this all might sound new, mRNA technology has been used in other vaccines for the last eight to ten years. During that time, it has been studied extensively with no long-term side effects or adverse consequences. While more research is needed, and being conducted every day, it is important to remember that mRNA vaccines are designed specifically to ensure that the recipient does not get sick from the virus – nor does it affect a person’s DNA. This is RNA, not DNA. It cannot alter genes. mRNA vaccines have simply created an amazing opportunity to safely achieve vaccination with an extraordinarily low chance for risk.

How are the Pfizer and Moderna COVID-19 vaccines different?

Essentially, there is no difference between the two vaccines. They are both mRNA vaccines. They both code for the spike protein of the virus. They both limit the sequence to only what is called a few base pairs. They both are packaged to be injected in fat globules. Where they differ, however, is the amount of salts and buffers in them. And this has to do with cold storage. Currently, the Pfizer vaccine needs to be stored at a colder temperature than the Moderna vaccine. Both the Pfizer and Moderna vaccines need to be stored at very cold temperatures, because mRNA is unstable. If it warms up too much, it won’t survive long enough in the human body to be effective. It has to be kept cold to maintain its stability and effectiveness. While mRNA vaccines – like the Moderna and Pfizer vaccines – are very effective, the cold storage is the one downside. Although Pfizer’s vaccine previously had to be kept at extremely cold temperatures, a recent study has demonstrated that the Pfizer vaccine can be stored at freezer temperatures similar to the Moderna vaccine. However, when it comes to protecting a person from contracting COVID-19, both vaccines are equally safe and effective.

What other vaccines are coming beyond the Pfizer and Moderna vaccines, and will they be safe or safer?

Many vaccines are currently being studied by several pharmaceutical companies – some of the most notable are:

Johnson & Johnson Vaccine: Unlike the Moderna and Pfizer vaccines (which require two shots), the Johnson & Johnson vaccine requires only one shot. In clinical trials, this vaccine was 72% effective against mild disease, 85% effective against severe disease, and no one was hospitalized or died 28 days after immunization. This vaccine should be available in the US very soon.

Novavax Vaccine: This US company has produced what is called a subunit vaccine. This vaccine does not contain live components of the pathogen, only the antigenic parts of the pathogen which are necessary to elicit a protective immune response. This vaccine is currently in phase three of clinical trials, showing overall efficacy of 90% with very promising results in combatting the more transmissible South African variant.

AstraZeneca/Oxford Vaccine: This vaccine has already been approved in the United Kingdom. It is a viral vector vaccine, which means it uses a modified version of a different virus as a vector to deliver instructions, in the form of genetic material (a gene), to a cell. Like the current COVID-19 vaccines available in the US, this type of vaccine does not cause infection with either COVID-19 or the virus that is used as the vector. This vaccine has been observed to be 62% effective and is not currently available in the US.

Are the current COVID-19 vaccines available in the US safe for people with compromised immune systems?

Whether it be radiation, chemotherapy, or any other circumstances that might potentially compromise an immune response – such as transplant rejection medication, HIV or AIDS, etc. – it is still important to get the COVID-19 vaccine. First, there is no danger in getting the vaccine, as it cannot give a person the virus. Secondly, even if a person’s immune response might not be as robust as others – any response is better than no response. The potential risks of receiving the vaccine – which are virtually none – far outweigh the risks involved with actually contracting COVID-19.

How about people with allergies?

One of the main things to consider about the COVID-19 vaccine, as it relates to allergy risks, is whether a person has a history of allergic reactions to the components of vaccines in general. Such allergic reactions are usually to a substance called polyethylene glycol, a common ingredient in all vaccines. If a person is known to have any reaction to this substance, that would certainly be a reason to defer vaccination. Outside of that, there is virtually no allergy risk related to the COVID-19 vaccine.

Are the current COVID-19 vaccines safe for women who are pregnant, trying to become pregnant, or breastfeeding?

According to the American Pediatric Society and the Advisory Committee on Immunization Practices (ACIP), if a woman is breastfeeding it is still advised to receive a vaccination. The vaccine, in and of itself, is not a risk to the baby. In fact, when a mother receives the vaccine, her breast milk then has antibodies that can be passed onto the child, protecting the child indirectly.

Unfortunately, the vaccine has not been studied extensively in pregnant women. However, out of the first million doses administered to the public, about 580 of them were given to pregnant women – and the number is growing every day. So, while there’s not much information yet, it is coming. That being said, from a medical perspective, if a woman is pregnant and considering the vaccine, the risks of contracting COVID-19 would be far more detrimental to an unborn child than the possible side effects of the vaccine.

Those who are considering becoming pregnant may be concerned about the impact of the vaccine on fertility – the question being how likely is it that the vaccine might affect the sperm or the ovum? The answer is, unlikely. Again, it is not a DNA vaccine; it is an mRNA vaccine. There’s no means for mRNA to interfere with spermatogenesis or oocyte formation and cause trouble with fertility or conception.

For those who are still hesitant about immunization, why is getting the COVID-19 vaccine so important?

There is always hesitancy with any vaccine, and with the COVID-19 vaccine in particular, the fear of getting vaccinated is very high. This hesitancy is for a myriad of reasons ranging from anxiety surrounding the pandemic to the newness of the vaccines and lack of any long-term research. However, as stated earlier, the vaccines are safe, and they are effective. When weighing the decision to get vaccinated, keep in mind the risks of getting the vaccine as compared to the risks of contracting COVID-19. Also think of the general population and helping to end the pandemic through herd immunity.

What is herd immunity? The reason people need to get vaccinated.

When it comes to herd immunity, the “herd” that is being referred to is the entire human population. The goal of herd immunity is to get enough of the population, or the herd, immune to COVID-19 so that it is no longer able to be transmitted from person to person. In order for that to happen, not every person needs to be immune. In fact, only a certain portion of the population needs to be immune for transmission to become impossible. In the case of COVID-19, about 70-80% of the population needs to be immune to achieve herd immunity.

In theory, that number could be achieved naturally, without a vaccine. However, the death rate would unnecessarily continue to rise significantly throughout the process. The US has already recorded over 500,000 deaths since the start of the pandemic, and it is estimated that only 15% of the population has had COVID-19. To achieve herd immunity naturally, more than five times the population would need to become infected, perhaps resulting in millions of more deaths. That is an untenable way to think about achieving herd immunity and that’s why getting vaccinated is so important. It is the best and safest way to achieve herd immunity, bringing an end to the pandemic, while saving as many lives as possible.


Dr. Ronald Nahass is a board-certified physician with over thirty years of experience in infectious disease. He is currently the hospital epidemiologist at Robert Wood Johnson University Hospital Somerset in New Jersey. In that capacity, he has been responsible for the oversight of infection prevention, and in this particular year, oversight of the COVID-19 medical team.

As President of ID Care, Dr. Nahass and his network of infectious disease specialists have treated over 20% of the hospitalized cases of COVID-19 in New Jersey. As the largest independent infectious disease practice on the East Coast, ID Care is always up-to-date on the latest information concerning COVID-19 vaccines and prepared to ease any possible concerns. If you’re interested in more coronavirus information and resources, visit 908-281-0610 to schedule a consultation with an ID Care expert.

COVID-19, Infectious Disease Blog