Everything You Need to Know About the Coronavirus Vaccine

By Amichai Perlman, PhD, PharmD
Medically reviewed
June 3, 2021

The coronavirus vaccine is a wildly exciting feat for the medical community and the world. But most people, understandably, have questions and concerns: what’s in the vaccine? Is it safe for children? Can you take it if you are pregnant? What if you have an allergic reaction to the vaccine?

Here, Dr. Amichai Perlman gives us answers to all of these questions and more.

Watch: Coronavirus Vaccine Myth Busting with Dr. Amichai Perlman


What is in the COVID-19 Vaccine?

The FDA approved three vaccines for emergency use against COVID-19. These vaccines train the body’s natural immune response to recognize, neutralize, and eliminate the coronavirus which causes COVID-19. 

The first two vaccines, developed by Pfizer-BioNTech and Moderna, are based on mRNA technology, and therefore contain synthetic mRNA designed to encode the instructions for a specific protein produced by the coronavirus called the “spike” protein. 

The third vaccine, developed by Johnson & Johnson, contains a benign and harmless virus called adenovirus (a “viral vector”). This virus is designed to insert DNA encoding instructions for the spike protein while being unable to replicate the virus itself in the human body.   

These vaccines do not contain preservatives or adjuvants. In addition to the mRNA or the viral vector, the vaccines contain various fatty acids, salts, and sugars. 

Find the exact ingredients of the Pfizer, Moderna and J & J vaccines.

What is mRNA and are all the vaccines employing this technology?

The first two vaccines authorized by the FDA for preventing COVID-19 contain mRNA, or messenger-RNA, which means that they have the ability to “deliver” instructions for creating specific proteins.

In the case of the coronavirus vaccine, the mRNA works as follows:

  1. The mRNA technology delivers instructions to the body to create one of the coronavirus proteins
  2. Following injection, these instructions are “read” by the ribosome in our cells
  3. The ribosome then creates copies of this viral protein and presents it on the cell wall
  4. The mRNA disappears in the body within days of injection, and the viral proteins are degraded within weeks
  5. In the meantime, our body’s immune system, which recognizes these proteins as foreign, develops specialized antibodies and T-cells for fighting the coronavirus

Just like that, the body is granted a new ability to fight off the coronavirus.

Extensive research has been conducted to study the effects of these vaccines, and they have been found to reduce the risk of COVID-19 by over 90%.

Other technologies being used for the development of COVID-19 vaccines include additional techniques to present the foreign protein to the body’s immune system. These include using a different benign virus (“vector”) which contains DNA encoding the coronavirus spike protein – this is the approach used in the Johnson & Johnson vaccine – and older versions of vaccine technology, which use the coronavirus itself in an inactive or attenuated form. 

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Why does the COVID-19 vaccine have to be kept so cold?

mRNA degrades quickly in room temperature, which means it must be stored and transported in very low temperatures.

The Pfizer-BioNTech is stored and transported at approximately -94℉ while the Moderna vaccine can be stored and transported at -4℉. The Johnson & Johnson vaccine is more stable and is stored at 36°F to 46°F. Before the vaccine is given, it is thawed and warmed to room temperature.

What companies are working on the coronavirus vaccine?

The first three vaccines authorized by the FDA for preventing COVID-19 were created by Pfizer-BioNTech, Moderna, and Janssen (a subsidiary of Johnson & Johnson). There are dozens of additional vaccines which are being developed around the world. Some of these are expected to report results and may be approved in the near future. 

How many people have had the coronavirus vaccine so far?

Since approval, over 300 million doses of the vaccine have been given – such that roughly 41% of the population in the United States are fully vaccinated.

When will a vaccine be available for you?

All U.S. adults age 12+ are now eligible for vaccination in the United States. You can find a vaccine site near you at https://vaccinefinder.org/.

Safety and side effects

What are the side effects of the COVID-19 vaccine?

The currently approved COVID-19 vaccines have been tested in large randomized clinical trials with tens of thousands of participants. The vaccines have been proven to be both safe and effective, and the reported side effects were characteristic to vaccines at large.

The most commonly reported side effects of the coronavirus vaccine included:

In the Moderna trial, swollen lymph nodes in the same arm as the injection, nausea and vomiting were common as well. These side effects typically lasted several days.

Note: the Moderna and Pfizer-BioNTech vaccines require two doses – more people experienced these side effects after the second dose than after the first dose.

Serious side effects were very uncommon in the initial vaccine studies. Some serious side effects were slightly more common among those receiving the vaccine.

However, the FDA has concluded that these side effects were not a result of the vaccine, but instead representative of medical events that occur in the general population at a similar frequency.

Serious adverse events considered by FDA to be plausibly related to the vaccine, or the injection procedure, were one case of shoulder injury at the vaccination site and one case of swollen lymph node in the armpit opposite the vaccination arm.

Following use of the vaccines by many millions of people, the vaccines have been linked to very rare cases of severe allergic reactions (see below). In addition, the J&J vaccine has been linked to severe cases of blood clots. The FDA & CDC have concluded these clots are very rare and the benefit of the vaccine far outweighs this risk, and have added information on this side effect to the vaccine information sheet. Additional adverse reactions, some of which may be serious, may become apparent with more widespread use of these vaccines.

The CDC is currently evaluating reports of myocarditis (heart inflammation) following vaccination with mRNA vaccines. The events  are rare and according to the CDC the benefits of the vaccines continue to outweigh known and potential risks. The majority of reports were not severe, recovered quickly, and occurred among male adolescents and young adults. Additional adverse reactions may become apparent with more widespread use of these vaccines.

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Can I get COVID-19 from the vaccine itself? 

The recently approved COVID-19 vaccines do not contain the coronavirus, and the virus is not used at any stage of production of these vaccines. This means that you cannot get COVID-19 from the vaccine.

The mRNA vaccines do not contain a live virus. Instead, they contain synthetic mRNA molecules which instruct the cells in our body to produce one specific viral protein for a short period of time. The Johnson & Johnson vaccine contains a harmless common virus, which cannot replicate in the human body and contains DNA for the same viral protein.

This protein is not enough to cause infection by itself, so the vaccines cannot cause COVID-19.

Can you still get COVID-19 after getting vaccinated?

Currently approved COVID-19 vaccines have been proven to reduce the risk of contracting COVID-19 illness by around 95%.  The Johnson & Johnson vector vaccine reduced COVID-19 by about 66%, and reduced severe illness by 85%. This means the chance of having the illness caused by the coronavirus is far lower following vaccination. However, there is still a small chance that you can get coronavirus post-vaccination.

How did they develop the vaccine so quickly? Did they cheat, cut corners, or compromise safety?

Researchers, clinicians, and regulators worldwide have scrutinized the COVID-19 vaccine data throughout the development of the newly approved vaccines. All approved vaccine undergoes several stages of rigorous testing, including:

  • Pre-clinical testing in the laboratory and in animals evaluating the potential usefulness and toxicity of the vaccine
  • Phase 1 clinical trials for initial evaluation of the safety of the vaccine in humans
  • Phase 2 trials evaluating the effect of different doses in humans based on laboratory markers
  • Phase 3 trials where the vaccine is tested in thousands of people to fully evaluate the vaccine’s actual effectiveness in preventing disease, and to gather more safety data

The FDA-approved vaccines for COVID-19 underwent all of the standard testing of vaccines.

The accelerated rate of these vaccine developments can be attributed to a number of factors. Perhaps the most notable one, known as “Operation Warp Speed,” was the US Government’s decision to take on the financial risk of vaccine development.

By covering the costs of promising vaccine candidates in advance, vaccine developers were free to invest in production and expensive clinical research without needing to spend time raising capital or carefully weighing the risk of failure at each stage of development.

A second important contributor was the pre-existing research into coronaviruses, and the decades of research developing the mRNA vaccine technology.

Lastly, bureaucratic processes were expedited and regulatory reviews were initiated in parallel with vaccine development, instead of only initiating the application and the review of all stages of research following the completion of all of the studies.

Is the coronavirus vaccine safe for my children?

Studies of vaccines for COVID-19 in children are currently underway. The Moderna and J&J vaccines have so far been tested and approved from age 18.

The Pfizer vaccine has been tested and approved from age 12, following results showing the vaccine is very effective and safe in these ages as well.

Side effects among adolescents receiving the Pfizer vaccine were similar to those reported in adults and typically lasted 1-3 days.

The most commonly reported side effects were pain at the injection site, tiredness, headache, chills, muscle pain, fever, and joint pain.

Serious adverse events were rare, and included one case of abdominal pain and four cases of depression. The FDA found no evidence these events were related to the vaccine. 

The CDC is currently evaluating reports of myocarditis (heart inflammation) following vaccination with mRNA vaccines. The events  are rare and according to the CDC the benefits of the vaccines continue to outweigh known and potential risks. The majority of reports were not severe, recovered quickly, and occurred among male adolescents and young adults. Additional adverse reactions may become apparent with more widespread use of these vaccines.

Allergic reactions & other medical concerns

Could I have an allergic reaction to the coronavirus vaccine?

Allergic reactions occur when the immune system overreacts to a harmless substance known as an allergen. This can happen in response to many substances, including pollen, foods, latex, mold, pet dander, dust mites, or insect stings. Allergic reactions can occur as the result of medications and vaccines as well, though they are typically a rare occurrance.

During the clinical trials of the currently approved vaccines,  immediate severe allergic reactions were only observed in the Johnson & Johnson trial. In addition, between 0.5-1.5% of participants reported milder reactions such as rash at the injection site, which could be related to allergies.

Since the rollout of the vaccine, the CDC has been tracking cases of anaphylaxis – a severe allergic reaction – in recipients. After the first 17.5 million doses:

  • 66 reactions were determined to be anaphylaxis.
  • 52 of these allergic reactions occurred in persons with a history of allergies or allergic reactions, seven of whom had a history of anaphylaxis.
  • Among 61 persons with follow-up information available, all had recovered or been discharged home.

Due to these cases, the CDC currently advises that the mRNA vaccines should not be given to people with a history of a severe or immediate allergic reaction to the mRNA COVID-19 vaccines, or to any of their components – most notably to “polyethylene glycol” (PEG) or to a similar compound called “polysorbate.”

People with a history of immediate allergic reaction to other vaccines or injections can receive the vaccine, but should consult with a physician to discuss and assess the risk for a severe allergic reaction and remain under observation following vaccination for 30 minutes.

According to the CDC, people with allergic reactions to other substances not related to vaccines or injections can receive the vaccine. If they have a history of severe allergic reaction they should remain under observation for 30 minutes post-vaccination.

Can you get the coronavirus vaccine while you are pregnant?

The potential risks of COVID vaccines during pregnancy are not definitely known because these vaccines have not been studied during pregnancy.

Based on current knowledge, experts believe the currently approved vaccines are unlikely to pose a risk to the pregnant person or the fetus.

Studies in pregnant women are planned, and the vaccine manufacturers are following outcomes of women who became pregnant during the clinical trials.

The use of the currently approved vaccines is permitted during pregnancy, according to the FDA, CDC, and the American College of Obstetrics and Gynecology.

According to the CDC, when making a decision regarding vaccination, pregnant women and their healthcare providers should consider all of the following factors:

  • The level of COVID-19 community transmission
  • The patient’s personal risk of contracting COVID-19
  • The risks of COVID-19 to the patient and potential risks to the fetus
  • The efficacy of the vaccine
  • The side effects of the vaccine
  • The lack of data about the vaccine during pregnancy

There is no recommendation for routine pregnancy testing before receipt of a COVID-19 vaccine, and those who are trying to become pregnant do not need to avoid pregnancy after COVID-19 vaccination.


Read On: Can the COVID-19 Vaccine Cause Infertility?


How do we know that 10 years from now I won’t have cancer because of this vaccine?

Cancer occurs following cumulative changes to cell DNA which lead to uncontrolled cell replication.

The approved mRNA vaccines do not enter the cell nucleus, or interact with or change the DNA. The other components of the COVID-19 vaccines are well known substances that are not expected to have carcinogenic potential.

The Johnson & Johnson viral vector vaccine uses a known adenovirus. This type of virus is extremely common and does not cause cancer in humans. While the viral DNA enters the cell nucleus it does not interact with or change the human DNA. 

Cases of cancer are recorded in the clinical studies, and while the follow-up period is short, there is no difference in the rate of cancer between participants who received the vaccine and those who received placebo.

Is the vaccine going to change my DNA?

The first two vaccines authorized by the FDA for preventing COVID-19 are mRNA vaccines. Unlike some DNA based gene therapies, mRNA vaccines never enter the nucleus of the cell, which is where our DNA is kept, and cannot change a person’s DNA.

Human cells do not contain enzymes which convert the viral mRNA into DNA, and the mRNA molecules degrade and disappear within hours following injection.

The Johnson & Johnson viral vector vaccine uses a known adenovirus. This type of virus is extremely common and does not cause cancer in humans. While the viral DNA enters the cell nucleus it does not interact with or change the human DNA.

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Will these vaccines cause autoimmune disorders?

In short: these vaccines have not shown any increased risk of autoimmune disorders. 

The currently approved COVID-19 vaccines cause the cells in our body to produce a specific viral protein for a short period. This protein is identified by the immune system as foreign and generates an immune response so that the immune system learns how to produce antibodies and immune cells against this protein which can neutralize the virus. 

The immune system is adept at differentiating between our own proteins and cells and those which are foreign. As a result, exposure to foreign proteins and organisms, including the coronavirus itself, usually does not generate an autoimmune response. 

The vaccines have been tested on animals and humans, including large clinical trials with tens of thousands of participants and the data did not indicate an increase in autoimmune or inflammation related conditions.

Should you get vaccinated?

Who should not receive the vaccine?

The vaccines should not be given to individuals with a known history of an immediate or severe allergic reaction (e.g., anaphylaxis) to any component of the vaccines, including polyethylene glycol (PEG) and a related compound called polysorbate.

I’m young and not in a risk group. Why should I get vaccinated?

The currently approved COVID-19 vaccines have been tested and approved for use in adults of all ages (Moderna and Johnson & Johnson from age 18, and Pfizer-BioNTech from age 12).

The trials evaluating these vaccines included tens of thousands of participants, and the vaccines have been found to be both safe and effective in reducing COVID-19 in all age groups.

While younger adults have a much lower risk of severe COVID-19 disease compared to older adults, many do experience significant, and sometimes serious disease, as well as long term complications from coronavirus. Getting the vaccine can minimize your risk of incurring the virus and its unwanted side effects.

In addition, the vaccine will reduce the spread of COVID-19, and thereby protect others in the community who are at greater risk.

Finally, widespread vaccination is expected to be a major element of ending the current pandemic and gradually resuming many of the activities. The sooner everyone gets vaccinated, the sooner we can all return to life as normal.

Can you still carry and transmit the coronavirus after vaccination?

The clinical trials testing the vaccines were not designed to measure carriage and transmission of the virus in people without symptoms.

Several studies conducted since vaccine rollout have established that the vaccines do reduce the spread of coronavirus, and demonstrated that fully vaccinated people are much less likely to be infected or to transmit the virus.

Based on this assessment, and the lower risk of severe disease among vaccinated individuals, the CDC has issued guidance relaxing most of the COVID-19 precautions for fully vaccinated people.


People are considered fully vaccinated 2 weeks after their second dose in a 2-dose series, like the Pfizer or Moderna vaccines, or 2 weeks after a single-dose vaccine, like Johnson & Johnson’s vaccine. If you don’t meet these requirements, regardless of your age, you are not fully vaccinated. Keep taking all precautions until you are fully vaccinated.


According to this guidance, if you are a fully vaccinated person you can resume activities that you did prior to the pandemic without wearing a mask or staying 6 feet apart, unless required to by local guidance.

In addition, you do not need to stay away from others or get tested, after being around someone who has COVID-19, unless you have symptoms or work in a correctional or detention facility or a homeless shelter. Likewise, if you travel inside the United States, you do not need to get tested before or after travel or self-quarantine after travel.

However, the vaccines are not going to eliminate infections entirely, and will not reduce transmission entirely. You should still watch out for symptoms of COVID-19, especially if you’ve been around someone who is sick.

If you have symptoms of COVID-19, you should get tested and stay home and away from others. You will also still be required to wear a mask on planes, buses, trains, and other forms of public transportation. Some workplaces and businesses may likewise continue to require precautionary measures.

Lastly, people who have a weakened immune system, should talk to their healthcare provider to discuss their activities and the need for additional precautions.

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.