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What’s the difference between a booster shot and a third shot?

When the Pfizer and Moderna vaccines came out, patients had to get two doses of the vaccine at least 21 days apart to be considered “fully vaccinated.”

For most people that second shot was enough.

Now, the CDC says that people who are immunocompromised — who have a weakened immune system — need to get a third shot to be fully vaccinated.

While many people only needed two, these patients need three to make sure they are protected.

While the third shot is about making sure everyone can become fully vaccinated, booster shots help to make sure you stay that way.

Certain kinds of vaccines weaken over time. For example, many patients get the Tdap vaccine, which protects against tetanus, diphtheria and pertussis, when they are about 11 or 12 years old. Because the Tdap vaccine weakens over time, the CDC says that adults should get a booster shot every 10 years to make sure they continue to be protected.

On Wednesday, the U.S. Department of Health and Human Services announced that the Pfizer and Moderna COVID-19 vaccines also weaken over time.

A team of U.S. health leaders issued the following statement:

The available data make very clear that protection against SARS-CoV-2 infection begins to decrease over time following the initial doses of vaccination, and in association with the dominance of the Delta variant, we are starting to see evidence of reduced protection against mild and moderate disease. Based on our latest assessment, the current protection against severe disease, hospitalization, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout. For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability.

This explains that the Pfizer and Moderna COVID-19 vaccines do weaken after you get your initial two or three doses.

Health leaders expect that people who got the Johnson & Johnson vaccine will need booster shots, but they are awaiting further data before making a decision.

Who needs to get a third vaccine shot?

According to the CDC, the only people who would need to get a third COVID-19 vaccine shot to supplement their initial two would be people who are moderately to severely immunocompromised.

This is because these patients are more at risk of serious, prolonged illness and especially vulnerable to COVID-19.

This includes people who have:

  • been receiving active cancer treatment for tumors or cancers of the blood.
  • received an organ transplant and are taking medicine to suppress the immune system.
  • received a stem cell transplant within the last two years or are taking medicine to suppress the immune system.
  • moderate or severe primary immunodeficiency (such as DiGeorge syndrome, Wiskott-Aldrich syndrome).
  • advanced or untreated HIV infection.
  • active treatment with high-dose corticosteroids or other drugs that may suppress your immune response.

The CDC reports that about 3% of the adult population would qualify for a third shot.

Anyone who thinks they may need a third shot should talk to their healthcare provider about their medical condition to determine if an additional dose is appropriate for them.

When should I get my third shot?

CDC recommends that people with moderately to severely compromised immune systems receive an additional dose of mRNA COVID-19 vaccine at least 28 days after a second dose of the Pfizer or Moderna vaccine.

Who needs to get a booster shot?

Everyone who got the Pfizer or Moderna vaccines. The CDC has confirmed that these COVID-19 vaccines weaken over time, like many other vaccines, so if you got fully vaccinated, you will need to get a booster shot to make sure you continue to be fully vaccinated.

Health leaders expect that people who got the Johnson & Johnson vaccine will need booster shots, but they are awaiting further data before making a decision.

When should I get my booster shot?

The health leaders say that Americans should get a booster shot eight months after their last dose.

Providers are expected to begin offering booster shots starting the week of Sept. 20.

“At that time, the individuals who were fully vaccinated earliest in the vaccination rollout, including many health care providers, nursing home residents, and other seniors, will likely be eligible for a booster,” the statement reads. “We would also begin efforts to deliver booster shots directly to residents of long-term care facilities at that time, given the distribution of vaccines to this population early in the vaccine rollout and the continued increased risk that COVID-19 poses to them.”