This information was reviewed and approved by Carrie A. Horn, MD (February 2021).
If you receive either of the two COVID-19 vaccines currently authorized for use in the United States, you will need BOTH doses given three to four weeks apart. Why two shots?
All vaccines work by exposing you to an infectious organism or a piece of it. Your immune system recognizes it as a potentially dangerous invader and mounts a multi-prong defense against it. When the immune system encounters that germ a second time, it is ready to mount a faster and stronger defense that prevents or reduces severity of illness.
Most vaccines require booster shots to work effectively. For example, the measles, mumps and rubella (MMR) vaccine requires two doses. Data show that people who get only one dose are four times more likely to catch the measles.
As with other vaccines, including the first dose of the Moderna or Pfizer-BioNTech COVID-19 vaccines acts as a “primer” for the immune system. It provokes the body to create some initial antibodies and immune cells against the SARS-CoV-2 virus. That happens within about one to two weeks after the first shot. These initial antibodies are weaker and shorter lived than those created by the second dose.
On September 22, 2021, the FDA amended the emergency use authorization (EUA) for the Pfizer-BioNTech COVID-19 Vaccine to allow for use of a single booster dose, to be administered at least six months after completion of the primary series in individuals 65 years of age and older; individuals 18 through 64 years of age at high risk of severe COVID-19; and individuals 18 through 64 years of age whose frequent institutional or occupational exposure to SARS-CoV-2 puts them at high risk of serious complications of COVID-19 including severe COVID-19. See Pfizer Fact Sheet.
There is some speculation and research being done to determine if those who have been infected with COVID-19 in the past may already have had their system primed, replacing the work done by the first dose of vaccine. At this time, there is no conclusive evidence that this is the case, but if proven, it may allow some people to only need a single dose of a two-dose vaccine.
The second dose greatly increases the number of immune cells and their ability to make antibodies against the virus. The vaccines reach their full effectiveness about 10 days to two weeks after the second shot. Both the Moderna and Pfizer-BioNTech vaccines have been shown to be about 95 percent effective at preventing symptomatic COVID-19 when both doses are given.
Many people feel soreness at the injection site, fever, achy muscles and other flu-like symptoms. They are not getting sick. The symptoms are caused by the immune system as it responds to the vaccine and are a sign that your body is developing strong protection against the virus.
It is not clear how long the vaccine will protect against the disease, although initial research indicates that antibodies and protective immune cells remain in the body for several months after vaccination and maybe much longer. Future research will shed light on the duration of protection provided by the vaccines.
While the vaccines have been shown to be 95% effective, it is not clear if the vaccines prevent 95% of all COVID-19 cases. This is a tricky, but important distinction. Some people might develop mild COVID-19, which they could spread to others. Others may carry the virus with no symptoms at all, but still be able to pass it on to others. That is why you should continue to wear a mask, wash your hands frequently and maintain social distance until your vaccine is fully effective. Although the CDC masking guidelines have eliminated the need for fully vaccinated people to wear a mask in most places, many settings, including health care, still require masks. Please continue to follow government, workplace and business masking guidelines.
In the year that the virus has been circulating around the world, it has mutated in ways that make some variants more effective at infecting people and possibly causing more severe disease. So far, the Moderna and Pfizer-BioNTech vaccines are mostly effective against all recognized variants, though the Delta variant is proving a challenge as it is much more infectious than original strains. It produces more virus and binds much quicker to the body.
Both Pfizer-BioNTech and Moderna are closely monitoring the emergence of new variants and the effectiveness of their vaccines against them. They are prepared to modify the vaccines, if needed, to assure that they are effective against new variants.
The FDA amended the emergency use authorizations for the Pfizer-BioNTech and Moderna COVID-19 vaccines on August 12 to allow for an additional dose, following an initial two-dose, for certain individuals who are severely immunocompromised. This is the exact same dose as the first two and is still a match for the Delta variant.
While fully vaccinated people still have immunity, a third dose wakes up their immune systems to prepare them for a new exposure. Most of our bodies can respond to the virus eventually, but a third dose will give us a boost to fight it faster, so we are less likely to develop symptomatic disease. You can read more the third dose here.
It is important to remember that the vaccine is significantly less effective after only one dose. After the second dose, it will take an additional two to three weeks for you to be fully protected. Even once you have the vaccine’s full immunity, you can still transmit virus to others who are not protected and even develop mild COVID-19 yourself.
Please continue to practice this safe behavior, even after you have been vaccinated:
Learn more about COVID-19 and how it affects specific health conditions in these printable patient education materials.
Download COVID-19 Materials