The COVID-19 vaccine booster helps the body fight the SARS-CoV-2 virus, which causes COVID-19. The booster may also reduce the risk of developing a symptomatic infection. However, people can still test positive after a booster shot, even if they have no symptoms.

It is a myth that the vaccine itself causes a positive test. The vaccine does not contain the live SARS-CoV-2 virus that causes COVID-19, and it cannot cause an infection.

A booster dose of the COVID-19 vaccine provides greater protection against the disease than the original series of vaccinations. This can reduce the risk of developing symptoms, but its chief purpose is to significantly reduce the risk of severe disease, hospitalization, and death.

If a person contracts SARS-CoV-2 after getting the booster, they may test positive. However, they may have a very mild case of COVID-19 or even an asymptomatic infection.

Read on to learn about how the booster works and what it means if a person tests positive. We also explore the side effects of the booster and more.

A person swabbing their nose for a COVID-19 test.Share on Pinterest
/Getty Images/Stocksy Irina Efremova/Stocksy

The booster significantly decreases a person’s chance of contracting the SARS-CoV-2 virus. Despite this, it is still possible to develop COVID-19 after the booster, so an individual may test positive.

According to a 2022 research letter, various studies suggest the odds of developing COVID-19 are 55% to 99% lower among people who have the COVID-19 booster. However, these studies draw on data from before the advent of the new booster, so the actual figures may be slightly different.

Some common scenarios for testing positive after a booster include the following:

  • A person might contract SARS-CoV-2 before the booster, then test positive a few days later.
  • An individual might contract SARS-CoV-2 shortly after the booster but before the booster becomes fully effective.
  • A person might contract SARS-CoV-2 even when fully boosted. In this scenario, the booster may still reduce the odds of severe illness. This is the main purpose of the booster.

The side effects of the COVID-19 booster are similar to those of the original COVID-19 vaccine side effects.

In 2021, the Centers for Disease Control and Prevention (CDC) published data on the side effects of the third COVID-19 shot and the original booster. In the week after vaccination, 74.9% of vaccine recipients reported local symptoms such as soreness at the injection site.

Systemic symptoms, such as fever and headache, were also common, with 69.9% reporting at least one such reaction.

According to the Food and Drug Administration (FDA), data from studies of the newest booster have found similar side effects to the older vaccine. The most common side effects include:

  • swelling, redness, and pain at the site of the injection
  • headache
  • fever
  • joint pain
  • muscle pain
  • chills

Rarely, a person may experience a severe allergic reaction. People with a history of severe allergic reactions to any component of the COVID-19 vaccine should not take the vaccine. However, there are very few medical reasons not to get the COVID-19 vaccine and boosters.

The newest COVID-19 booster provides protection against newer variants of SARS-CoV-2, including some types of Omicron. However, researchers do not have a precise figure on how long it offers protection.

Most research suggests that protection against disease declines fairly quickly, but protection against severe infection, hospitalization, and death lasts longer.

For example, a 2022 CDC analysis found that a third COVID-19 booster had 87% effectiveness against emergency department and urgent care visits. It also had 91% efficacy against hospitalization in the 2 months following the vaccine. By 4 months, those figures declined to 66% and 78%, respectively.

The CDC gathered this data before the newer booster became available. This means the data for that booster may be different but are not yet available.

It is also important to note that the specific risk of disease, hospitalization, and other outcomes depend on many factors. A 2022 study using data from before the widespread dominance of Omicron found that fully vaccinated individuals who died from COVID-19 were more likely to be:

  • older, with a median age of 82
  • living in a long-term care facility
  • experiencing at least one underlying medical condition associated with a higher risk of death

People can still develop COVID-19 after receiving the booster. Contracting SARS-COV-2 is a breakthrough infection.

When a person gets a breakthrough infection, their symptoms are more likely to be mild and they are significantly less likely to become severely ill.

Sometimes, people test positive for COVID-19 after a booster and do not have symptoms. This is an asymptomatic infection. It may also be a presymptomatic infection, which is a positive test before symptoms begin.

People with an asymptomatic infection can still spread the virus. However, researchers do not know the extent to which they can spread the virus or the specific role they play in its spread.

The new bivalent booster offers protection against now dominant Omicron variants.

According to the CDC, people should seek a bivalent booster if it has been more than 2 months since their last monovalent booster. People who have recently had COVID-19 may wait up to 3 months after their most recent infection before taking the booster.

Even after having COVID-19, a person can still contract the virus. However, a vaccine offers additional protection.

It is still possible to test positive for COVID-19 after having a booster. This does not mean that the vaccine caused the positive test or did not work.

The main goal of the vaccines is to prevent severe disease, hospitalization, and death. These negative outcomes are very rare in fully vaccinated individuals.

People with questions about COVID-19 vaccines should consult a healthcare professional.