UNC Health Talk

What We Know (and Don’t Know) About the COVID-19 Vaccines

After weeks of exciting news about the promising results of the Pfizer and Moderna coronavirus disease 2019 (COVID-19) vaccine trials, a small subset of Americans is actually receiving vaccines. This is a momentous occasion because clinical trials show the first two vaccines are about 95 percent effective at preventing people from getting sick with COVID-19.

With a continued surge in cases in the United States and around the world, the news about the efficacy of these two vaccines is encouraging. We talked to UNC Health infectious disease expert Cynthia Gay, MD, MPH, who leads the Moderna clinical trial at UNC, about what we know and don’t know about the vaccines.

What We Know:

1. The COVID-19 vaccines require two doses.

Vaccines currently prevent millions of deaths every year from diseases such as diphtheria, tetanus, pertussis, measles and influenza. They work by training and preparing the body’s natural defenses – the immune system – to recognize and fight off the viruses and bacteria they target. This way, if the body is exposed to those disease-causing germs later, the body is immediately ready to destroy them, preventing illness or decreasing the severity of illness. Vaccines are also critical to the prevention and control of infectious-disease outbreaks.

The COVID-19 vaccines are administered as a shot into the muscle of the upper arm. A second dose is then administered either three or four weeks later depending on the vaccine.

“The reason that the COVID-19 vaccines have two doses is that the earlier data show that the second dose improves the immune responses as opposed to just one,” says Dr. Gay.

It is not uncommon for a vaccine to require multiple doses to receive maximum protection from a vaccine, and even if you already have had COVID-19, the U.S. Centers for Disease Control and Prevention (CDC) recommends you still get the vaccine. A prior COVID-19 infection may not indicate immunity to the disease in the future.

2. The vaccine trials adhered to all required evaluation steps.

The COVID-19 vaccine trials have been through every stage of clinical trials that would normally take place for a vaccine. The U.S. Food and Drug Administration (FDA) followed all required steps, and the CDC and the Advisory Committee on Immunization Practices (ACIP) have been actively involved in the evaluation process.

“It is very important to understand that it is just not possible to get a vaccine into humans without doing all the necessary steps,” says Dr. Gay. “It is a requirement that the FDA review a series of data initially in animal data, then phase I studies, then phase II studies, before we get to a phase III study.” An independent Data and Safety Monitoring Board also reviews all data and reports findings to the National Institute of Allergy and Infectious Diseases.

The vaccine development timelines were accelerated because teams of scientists all over the world worked together and around the clock.

“Early in the pandemic there was recognition by the people who spend their lives and careers thinking about vaccines and prevention that we really needed COVID-19 vaccines,” Dr. Gay says. “People with considerable expertise and leadership combined forces to eliminate some of the barriers and the time it normally takes to get a vaccine from development through to phase III study. But no safety steps were skipped.”

The primary barrier is usually funding. Typically, it takes time to get funding for each phase of a vaccine clinical trial because companies want to be sure candidates successfully pass through each step in the development process before investing funding into the next phase. Given the devastation caused by the pandemic, there was a vested interest among numerous companies and individuals to ensure funding was not an obstacle.

“If you removed the steps and requirements of having to apply for funding at many stages, you can accomplish a lot, and you can accomplish it a lot quicker,” Dr. Gay says.

3. The most common side effect is arm soreness.

The most common side effects of the vaccine are some tenderness, swelling, maybe some soreness at the injection site. Other symptoms include feeling fatigued or a mild headache.

“Those mild symptoms of feeling achy, (and) maybe some low-grade fever are really your immune system telling you that the vaccine is doing exactly what we want it do,” Dr. Gay says. “We want your immune system to respond to the vaccine (and) produce antibodies such that if and when someone is exposed to the coronavirus, those antibodies are present and can recognize, (and) bind to the coronavirus, and either prevent infection or make the infection less severe.”

So, the vaccine can make a person feel like they are sick, but symptoms almost always last just one to two days. 

What we don’t know are the possible long-term side effects, but our long history of making vaccines and the short-term safety data give confidence to scientists that the COVID-19 vaccine candidates are safe in the long-term. One reason is because the vaccine cannot enter the nuclei of our human cells and cannot cause a Covid-19 infection.

4. We still need to wear masks and practice social distancing.

Vaccines are one important tool in our toolkit to fight COVID-19, but we still need to use all of our tools if we hope to end this pandemic. This means it’s still important to adhere to the following safety measures:

What We Don’t Know

1. The long-term effectiveness of the COVID-19 vaccine is unknown.

As of late November, early results from multiple studies of COVID-19 vaccines show that for people who received both doses of the vaccine, the vaccine is about 95 percent effective at preventing people from getting sick with COVID-19. This means there were 95 percent fewer people with COVID-19 in the study groups getting the vaccine compared to the groups who received a placebo.

However, we do not know the long-term ability of COVID-19 vaccines to prevent infection at this time. It is possible that you will need additional doses (boosters) of the vaccine in the future similar to the way the flu vaccine works.

Researchers will continue to follow the vaccine study participants to determine how long the vaccines might be effective.

“The reason to follow people for 12 months to two years after receiving the COVID-19 vaccines is because we want to know what the durability of the immune response would be, meaning, is this a similar scenario to influenza where we might have to, for example, give a yearly booster vaccine?” Dr. Gay says.

2. Vaccine effectiveness in children has not yet been evaluated.

Children, pregnant women, breastfeeding women and those who are immunocompromised were not included in phase II or III of the clinical trials for the COVID-19 vaccines, so it is not known yet if the vaccine would be effective for these populations. Further study is needed.

3. The availability of the COVID-19 vaccine for the general public is still to be determined.

For now, vaccine supplies are limited and eligibility varies based on state. Generally, healthcare workers, frontline essential workers, people living in long-term care facilities and older individuals are being prioritized for the first available doses. Check your state’s health department website to find out when you and your family can get vaccinated. .


Click here for the latest information on the COVID-19 vaccines or visit the CDC website and the UNC Health COVID-19 Resources page, and follow UNC Health on Twitter, Facebook, Instagram and YouTube.