COVID-19 Vaccine Facts

COVID-19 Vaccine Effectiveness

January 7, 2021: Vaccines, in general, introduce a less harmful part of the germ — or something created to look or behave like it — into a person’s body. The body’s immune system develops antibodies that fight that particular germ and keep the person from getting sick from it. Later, if the person encounters that germ again, their immune system can “recognize” it and “remember” how to fight it off.


Vaccine is effective after 2nd dose

All COVID-19 vaccines currently available in the United States have been shown to be highly effective at preventing COVID-19Learn more about the different COVID-19 vaccines.


The vaccines that are currently available from Pfizer and Moderna need two shots to be most effective. The vaccines are 95% effective after both doses are received; the Pfizer vaccine 7 days after the 2nd dose and the Moderna vaccine 14 days after the 2nd dose. Pfizer studies showed that the vaccine is about 52% effective after the first dose, and 95% effective after the second dose. 


All COVID-19 vaccines in development are being carefully evaluated in clinical trials and will be authorized or approved only if they make it substantially less likely that a person will get COVID-19Learn more about how federal partners are ensuring COVID-19 vaccines work.


Masks and social distance still needed

While the vaccine may prevent illness, it is unknown at this time if a person can still carry and transmit the virus to others, even when fully vaccinated. Hence, we all will need to continue wearing masks and socially distancing. 


Vaccine may reduce the seriousness of the illness

Based on what we know about vaccines for other diseases and early data from clinical trials, experts believe that getting a COVID-19 vaccine may also help keep individuals from getting seriously ill even if they get COVID-19, and protect others, particularly people at increased risk for severe illness from COVID-19.


More studies about the effect of COVID-19 vaccination on the severity of illness from COVID-19, as well as its ability to keep people from spreading the virus that causes COVID-19 are underway.


Vaccine may need a boost

A few people who have had COVID-19 have apparently had a second, often milder case of the disease, and researchers are exploring what this means in terms of how long immunity from the coronavirus lasts. Vaccine developers are looking at ways to boost the effectiveness of a vaccine so that it provides longer immune protection than a natural infection with the coronavirus.


Vaccination is safest way to build immunity 

Illness from COVID-19 can have serious, life-threatening complications, and there is no way to know how COVID-19 will affect an individual. And if a person gets sick, they can spread the disease to friends, family, and others around.


Getting COVID-19 may offer some natural protection, known as immunity. But experts don’t know how long this protection lasts, and the risk of severe illness and death from COVID-19 far outweighs any benefits of natural immunity. COVID-19 vaccination creates an antibody (immune system) response without the person having to experience the illness.


Experts are still learning more about both natural immunity and immunity produced by a vaccine. CDC will share new evidence as it becomes available.


COVID-19 Vaccine Safety

Clinical trials of all vaccines must first show they are safe and effective before any vaccine can be authorized or approved for use, including COVID-19 vaccines. The known and potential benefits of a vaccine must outweigh the known and potential risks of the vaccine if it is to be used under an Emergency Use Authorization (EUA). Watch a video on what an EUA is.


U.S. vaccine safety system has oversight

Safety has been a top priority as federal agencies work with vaccine manufacturers to develop and authorize a COVID-19 vaccine. Key parts of COVID-19 vaccine development, review, and authorization include:

  • Careful testing. All vaccines go through clinical trials to test safety and effectiveness. For the COVID-19 vaccine, the Food and Drug Administration (FDA) set up rigorous standards for vaccine developers to meet. This infographic from the National Institutes of Health shows the four phases a vaccine must go through before it is released to the public.
  • Authorization for emergency use. Vaccines that meet FDA safety and effectiveness standards can be made available in the United States by approval or by emergency use authorization (EUA). An EUA provides temporary authorization of a vaccine or medication under emergency situations, such as the coronavirus pandemic.
  • Continuous monitoring for problems and side effects. After a vaccine is authorized for use, monitoring continues, with systems in place to track problems or side effects that were not detected during the clinical trials. For the COVID-19 vaccine, the FDA and the Centers for Disease Control and Prevention (CDC) are expanding their vaccine monitoring. If there are problems with the vaccine, they are most likely to emerge early in the testing process when they can be identified and addressed.

You can learn more from the CDC about the safety steps in place for the COVID-19 vaccine. John Hopkins Medicine also has information on the safety of the COVID-19 vaccines.


Some side effects are normal

Some individuals may have side effects, which are normal signs that the body is building protection. Common side effects include pain and swelling in your arm, which should go away in a few days. Other common side effects include pain and swelling at the injection site, and possible fever, chills, tiredness, or headache. In clinical trials, about 0.4% of participants experienced a serious adverse event. CDC is collecting reports on side effects at 


Few reactions to most allergies

According to the CDC, individuals with allergies to certain foods, insects, latex, and other common allergens can receive the COVID-19 vaccine. Those with a history of a severe allergic reaction (anaphylaxis) to injectables or other vaccines should discuss the vaccination with their doctor, who can evaluate and assess their risk. At this time, anyone who has a severe allergy (e.g., anaphylaxis) to any of the Pfizer/BioNTech vaccine ingredients should not receive the COVID-19 vaccine.

Vaccines use a new “messenger RNA” technology

Vaccines against viruses work by sending a message to the body to be on the lookout for a certain virus, such as SARS-CoV-2. Traditional approaches have used dead or weakened versions of the virus itself, which the body learns to identify and develop immunity to. Making new vaccines this way can take years.


The first two COVID-19 vaccines were created using a new technique called “messenger RNA”, or mRNA. The method has been in development for years before the pandemic, and the arrival of the new coronavirus provided vaccine manufacturers a chance to use it. The mRNA is synthetically made in a lab rather than taken from the virus directly as is the case with other vaccines. Learn more about how mRNA vaccines help the body boost the immune system from Dr. Anuj Mehta, National Jewish Hospital at


The mRNA approach works differently and maybe even better at protecting people from the disease than those made with dead or weakened viruses. When a virus such as SARS-CoV-2 enters the body, it sets in motion a means of producing copies of itself like a photocopy machine. The copies of the virus invade other cells. The mRNA coronavirus vaccines cause the body to produce copies of just one part of the coronavirus: the spike proteins on its surface that give the virus its telltale appearance. The spike protein helps the virus attach to cells and make people sick, but by itself, it cannot cause COVID-19.


The new mRNA vaccines for the coronavirus contain “instructions” for how to make copies of the spike protein. It is manufactured to be injected and to find its way into cells. In the cell, mRNA instructs the cell to make copies of the coronavirus’s spike proteins. When the cell releases these proteins, the immune system identifies them as foreign and destroys them, but not before making antibodies to detect and react to the protein and the virus that causes COVID-19. Then, later, if alive, the immune system “remembers” the spike protein and attacks the coronavirus so it cannot reproduce and make someone sick with COVID-19.


Vaccination Timing

Those who get COVID-19 after the first dose should still get a second dose

As long as the individual is no longer in isolation, meaning 10 days have passed since experiencing COVID symptoms, it is fine to get the second dose. 


Those with certain health conditions will receive vaccine sooner

Individuals with certain underlying health conditions are at greater risk for death and disability from COVID-19. These conditions are obesity, diabetes, chronic lung disease, significant heart disease, chronic kidney disease, cancer, and having a compromised immune system. 


Vaccination & COVID-19 Testing

Vaccination will not cause a positive COVID-19 test result

COVID-19 vaccines will not cause you to test positive on COVID-19 viral tests. If your body develops an immune response, which is the goal of vaccination, there is a possibility you may test positive on some antibody tests.


Additional Sources