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9 things to know about COVID-19 vaccine safety and development

Learn more about COVID-19 vaccine side effects, safety, and the coronavirus vaccine timeline for Massachusetts, Connecticut, Maine, and New Hampshire.

As of April 13, 2021, the CDC and FDA recommend pausing the Johnson & Johnson vaccine due to rare complications. Please see their official statement here. Harvard Pilgrim Health Care will update the information below as official decisions evolve. For information on the COVID vaccine and additional resources, please visit our Guide to Care.

While the beginning distribution phases of the COVID-19 vaccines are spreading hope and optimism for some, questions around everything from speed to market to efficacy and side effects are still swirling in the minds of many. Is it safe? Should I get it? If so, when can I get it?

With expert guidance from trusted medical sources, including Dr. Patricia Toro, MD, MPH, associate medical director of Harvard Pilgrim Health Care and an infectious disease specialist, we’ve sought to help answer these pressing questions and ease concern.

Here’s what you need to know right now about the COVID-19 vaccine development, safety, and efficacy.


1. What is the COVID-19 vaccine?

By definition, a vaccine is a substance that helps to protect against and prevent certain illnesses. Currently, there are two types of vaccines available for COVID-19, messenger RNA (mRNA) vaccines that include the Pfizer-Biotech and Moderna vaccines, and a viral vector vaccine, made by Johnson & Johnson:

mRNA vaccines

mRNA vaccines have been developed and implemented for earlier coronaviruses like SARS and MERS. Instead of putting weakened or activated germs in the body like traditional vaccines, the mRNA vaccine teaches cells how to make a protective protein. Should the real virus enter the body, that new protein triggers an immune response that produces antibodies and prevents us from getting sick.

Viral vector vaccines

Viral vector vaccines piggyback the instructions to fight the coronavirus onto a separate and different virus, called an adenovirus. The virus is not alive and therefore cannot infect the individual receiving the vaccination. Viral vector vaccines have already been used to fight a number of diseases, including Zika, influenza viruses, HIV, malaria, and Ebola.

Both vaccines had a blueprint

The existing “blueprints” from previous vaccines enabled vaccine manufacturers to develop the COVID-19 vaccine more quickly. Dr. Toro notes that this was one of several reasons they were able to develop these vaccines in “10 months versus 10 to 15 years.”

2. How do they work?

Both types of vaccines help create a protein in the body. More specifically, the COVID-19 vaccines instruct our cells to create the “spike protein”–a protein found on the surface of the virus that causes COVID-19. Here’s a breakdown of how the vaccines work in our bodies.

I. Through a shot injection to the arm, the vaccine enters the system and either the mRNA or vector instructions permeate the immune cells. (Note: Neither vaccine becomes part of our DNA because their makeup is different and can’t change genetic code. mRNA vaccines never enter the cell’s nucleus, and only are inside of cells for 72 hours before degrading. While vector vaccines do enter the nucleus, they too degrade over time.)

II. Prompted by either the mRNA or the vector, the immune cells create the spike protein. Once the spike protein is made, the body gets rid of the original mRNA instructions and displays the newly created protein piece on the cell’s surface.

III. Since immune systems don’t recognize this new protein, they go through the process of building an immune response and making antibodies, just like they would for a natural COVID-19 infection.

IV. Now bodies have learned to protect themselves against COVID-19 and we have an immune response ready to prevent getting infected with the real virus.

3. How were they developed and authorized?

All vaccines are required to go through extensive testing before they’re approved or authorized for emergency use by the U.S. Food and Drug Administration (FDA) and introduced to the public. With prioritized funding and resources, and trials running in parallel, the COVID-19 vaccines went through the same clinical development process as all other new vaccines, just in a much shorter time frame. The World Health Organization breaks down new vaccine development as follows.

Phase 1:

The vaccine is administered to a small number of volunteers to test for 1) safety, 2) efficacy (does it generate an immune response?) and 3) the appropriate dosage. In this phase, vaccines are typically tested on young, healthy, adult volunteers.

Phase 2:

Then the vaccine is given to several hundred more volunteers to further determine its safety and ability to generate an immune response. Participants in this phase generally have the same characteristics (such as age and sex) as the intended audience of the vaccine, and there are usually multiple trials to evaluate different age groups and formulations of the vaccine. At this stage, a control group (a group that gets a placebo instead of the real vaccine) is added to determine if changes in the vaccinated group are truly caused by the vaccine or occurred by chance.

Phase 3:

Next, the vaccine is administered to thousands more volunteers and compared to a control group to determine effectiveness and safety in a larger number of volunteers. Moderna enrolled about 30,000 volunteers while Pfizer enrolled a little more than 43,000, numbers Toro notes are typical for a Phase 3 trial. These trials are often conducted across several countries and sites within a country to confirm that the vaccine performance findings apply to different populations. Every Phase 3 study is reviewed by the Data Safety Monitoring Board, so that the clinical trial is stopped should the participants be harmed by the vaccine. For all of these vaccines, the Phase 3 studies were completed and safety was maintained to the end.

After a vaccine has completed all three clinical development phases, typically the company that has developed the vaccine will submit a Biologics License Application (BLA) to the FDA seeking approval to market and distribute in the U.S.

Recognizing the COVID-19 pandemic as a public health emergency, however, vaccine manufacturers instead submitted an Emergency Use Authorization (EUA) request to the FDA, which enables the vaccine to be distributed to the public prior to receiving FDA approval. In order to authorize an EUA, the FDA must first rigorously evaluate all the data available and then determine that the potential benefits outweigh the potential risks. After an EUA has been issued, as it has with the COVID-19 vaccines, both the vaccine manufacturer and the FDA continue to closely monitor the vaccines as more data becomes available.


4. Should I get vaccinated?

The CDC reports that current data shows the vaccines are highly effective. In every Phase 3 study for these vaccines, no individuals died of COVID-19 if they received the vaccine, which is a key metric of effectiveness. The mRNA vaccines prevented any kind of symptomatic infection about 95% of the time. Meanwhile, The Johnson & Johnson vaccine prevented any symptomatic infection 72% of the time in the U.S.

Getting vaccinated can protect you, and early evidence suggests that vaccination may decrease your risk of transmitting the virus if you are infected.

So, is it better to receive the mRNA vaccines?

No. The best vaccine to take is the one that’s available to you the soonest.

There are two contributing factors as to why the Johnson & Johnson vaccine results differ from the mRNA vaccine results:

• The Johnson & Johnson vaccine was tested in different countries than the mRNA vaccines, with different populations. In particular, it was tested in Brazil and South Africa while the variant coronaviruses were the dominant strains. The mRNA vaccines were not tested against variants.

• The Johnson & Johnson vaccine was also tested at a different time during the epidemic. The background transmission rate may have impacted the final efficacy results. On the most important metric, preventing illness so bad that an individual is either hospitalized or dies, the Johnson & Johnson vaccine was successful 85% of the time. And again, no one who received this vaccine died in the Phase 3 trial.

5. When can I get vaccinated?

Vaccine vials with a label that reads "Coronavirus vaccine"

While COVID-19 vaccines are currently available in all states, availability continues to be limited and priority groups’ eligibility is still being determined by each individual state. Use the following resources to stay in the know on the coronavirus vaccine timeline in your state:

Massachusetts: View rollout schedule; find a vaccination site

Connecticut: View rollout schedule; find a vaccination site

Maine: View rollout schedule; find a vaccination site

New Hampshire: View rollout schedule

Harvard Pilgrim members can stay up to date on information about vaccine availability and locations in our guide to COVID-19 care.

6. How many shots do I need to be fully vaccinated?

There are currently three vaccines authorized for emergency use in the U.S. Two of these vaccines, created by Moderna and Pfizer-Biotech, require two shots that will need to be taken at least a few weeks apart for maximum protection. The third vaccine from Johnson & Johnson requires only a single shot. These vaccines protect you fully approximately two weeks after receiving the final dose.

Once you receive the vaccine, the CDC has developed a smartphone app called v-safe that you can download to help monitor your health and remind you when you’re due for your second dose.

The CDC recently shared updated, less restrictive guidelines stating that individuals who have been fully vaccinated can:

• Visit with other fully vaccinated individuals indoors without wearing masks or physical distancing.

• Visit with unvaccinated individuals from a single household who are at low risk for severe COVID-19 disease indoors without wearing masks or physical distancing.

• Refrain from quarantine and testing following a known exposure if asymptomatic.

If you are in public, however, everyone should continue to wear a mask and continue physical distancing. And anyone who has symptoms should continue to be tested, no matter their vaccination status.


7. Are the vaccines safe? Are there side effects?  

These vaccines are safe. Just like with any vaccine, COVID-19 vaccine side effects can occur as your body starts to build protection and can include swelling or pain at the site of the shot, fevers, chills, tiredness, nausea, or headaches. These symptoms may last one to two days and are generally mild. This shows that the vaccine is working in your body.

Very rarely (about four times in one million injections) someone has a severe reaction to the vaccine. Out of caution for this, you may be asked to stay at the vaccination site for 15 to 30 minutes after your vaccine to be monitored. Any reaction can be easily treated by on-site health care providers.

“The COVID-19 vaccine is entering the phase where we will find rare events and complications, but we should not assume these are happening because the vaccine wasn’t tested. This happens with every vaccine when it’s administered to the public.”

–Dr. Patricia Toro, MD, MPH and associate medical director of Harvard Pilgrim Health Care

After you’ve been vaccinated, you can use the CDC’s v-safe app to get personalized health check-ins and report any side effects. If needed, a CDC professional will follow up to get additional information.

8. What about new strains, or “variants?” Will a vaccine protect me against them?

Yes. The Johnson & Johnson vaccine was specifically tested against the B.1.351 (“South African”) and P.2 (“Brazil”) strains and proved to be very protective. The Pfizer and Moderna vaccines are being tested more formally against these “variants of concern,” and we hope to know more soon. That said, communities that have been hit hard by these variants display decreased infections and hospitalizations when they are vaccinated by the mRNA vaccines.

Dr. Toro further explains that “if the variants mutate more and the vaccines become ineffective, then these mRNA vaccines can be ‘retooled’ within six to eight weeks to work specifically against the variants.” A “booster” shot against these variants is currently being tested for the mRNA vaccines.”

9) How long will the vaccines last?

Since the vaccines are so new, it’s unclear how long they will last until more data becomes available. That said, early data suggests that most people will be protected for at least eight months, and perhaps longer.

While there’s a lot we now know and can prepare for with the COVID-19 vaccines there is still so much that’s consistently changing. Over the next few months, remember to stay aware of updates from your individual state, as well as the CDC for new developments and changes in vaccine availability. And remember that we are still in a pandemic, so for the protection of yourself and others, wear a mask and keep up the social distancing.


Harvard Pilgrim Health Care and Tufts Health Plan have officially come together as of January 1, 2021. If you are a current Tufts Health Plan member seeking the most recent information about COVID-19 and the vaccine, please visit For Harvard Pilgrim Health Care members, visit


This content was written by the advertiser and edited by Studio/B to uphold The Boston Globe's content standards. The news and editorial departments of The Boston Globe had no role in its writing, production, or display.