EXPLAINER: To boost or not to boost: How can a third shot help?

A vial of Pfizer's coronavirus vaccine. (Photo by Spencer Davis/Unsplash)

By Sydney Wyatt
Boston University News Service

Back in September, the FDA approved the Pfizer-BioNTech COVID-19 vaccine booster for individuals age 65 and older, immunocompromised people at high risk for severe COVID-19 and those who are frequently exposed to COVID-19. However, per experts, the booster shot must be received within six months of completion of the initial vaccine doses. 

According to the New York Times’ “vaccinations by country” tracker, about 8.5 million people in the United States have received a booster shot as of October 13. 

To understand how the booster shot works, let’s first look at our immune response to the initial COVID-19 vaccination doses:

  • The Pfizer COVID-19 vaccine is a two-dose series. Multi-dose vaccines are common and most vaccinations recommended for children and many given to adults are received in multiple doses, including vaccines for hepatitis A and B, HPV, polio, and varicella.
  • With some medical conditions, two or more doses of a vaccine are required to reach immunity. With the COVID-19 two-dose vaccination, so far researchers are seeing that most individuals gain optimal immunity after their second dose. The Pfizer COVID-19 vaccines have shown to remain above 90% effective for at least six months.
  • The first shot teaches the immune system how to identify SARS-CoV-2, the virus that causes COVID-19. It does this by using messenger RNA, which delivers a genetic code to cells. These cells then produce proteins, known as SARS-2 spike proteins, that activate the immune system. After that, the body is more prepared to recognize and fight off COVID-19 pathogens, but for most people, that isn’t quite enough. 

The human immune system is incredibly complicated, and there are many different factors contributing to its response to infection. The second dose provides the extra protection needed to reach the body’s highest possible level of immunity, allowing the immune system to produce enough specialized cells to fight off the SARS-CoV-2 virus.

So, why get a booster shot?

A booster shot is just an additional dose of the COVID-19 vaccine. There are many reasons why someone might need a booster since the human immune system is very complex and its response may vary among individuals.

One reason someone might need a booster shot is if they did not respond sufficiently to the first two doses. This is more likely to occur in immunocompromised individuals whose immune systems are already working hard to battle other illnesses.

Another reason an individual may need a booster shot is if their immunity begins to wane over time. Immunity may decrease over time, which is especially common as people get older. 

A booster shot may also become more necessary in response to emerging variants. As of now, receiving two doses of the Pfizer COVID-19 vaccination is 93.7% effective against the Alpha variant and 88% effective against the Delta variant, a study in the New England Journal of Medicine found. 

Moderna is currently seeking emergency use authorization for their booster shot, which is half the dosage of the original two shots.

The FDA has also recently stated that the Moderna booster vaccine does not meet all of its criteria. Johnson & Johnson has also asked for FDA authorization of booster shots following the original dose but no response has been released yet.

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