This information was reviewed and approved by Lindsay Sense, RN, BSN, CMSRN (March 2022).
If you receive one of the COVID-19 vaccines currently authorized for use in the United States, you will need a second dose, given three to four weeks apart, to complete your primary vaccination. 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 versions, COVID-19 vaccines acts as a “primer” for the immune system. They provoke 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
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.
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, especially if you’re in a high-risk area.
Additionally, there is evidence of waning vaccine efficacy over time. To prevent this dip in protection, the CDC has recommended booster shots five months after the primary series for both the Moderna and Pfizer vaccines. You can learn more about booster shot eligibility with the CDC's latest chart.
The COVID virus is capable of mutations that produce variants, some of which are more infectious and/or life-threatening than the original strain. So far, the Moderna and Pfizer-BioNTech vaccines are mostly effective against all recognized variants.
Healthcare entities are closely monitoring the emergence of new variants and the effectiveness of their vaccines against them.
The FDA amended the emergency use authorizations for the Pfizer-BioNTech and Moderna COVID-19 vaccines on August 12, 2021, to allow for an additional dose, following the primary series. This booster shot is recommended to prevent the waning efficacy of the vaccine over time.
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:
The information on our website is medically reviewed and accurate at the time of publication. Due to the changing nature of the COVID-19 pandemic, information may have since changed. CDC.gov and your state’s health department may offer additional guidance.