The main benefit from COVID vaccines is the prevention of serious disease. Overall, about 95% of people getting vaccinated will directly benefit. This leaves 5% of vaccinated people who will remain at risk of infection and severe disease. In other words, if 300 million people receive two doses of the COVID vaccines, then approximately 15 million people may not be fully protected.

Antibodies after vaccine

After vaccination (or after an infection causing COVID), the body creates antibodies against the virus. The most important antibodies are called neutralizing antibodies (NAbs). Nabs work by blocking the binding of the spike protein on the virus to our cells.

We know that vaccines do not provide protective immunity in everyone. As a result, it is important to see what changes there are in Nab levels over time and with additional booster shots. In a new study, about 300 healthy people received either the Pfizer or Moderna vaccines. Their NAb levels were measured 2 – 4 weeks and 2 – 4 months after their second dose. Levels were measured again before and after a third vaccine dose.

In this study, 14% to 31% of people did not have a significant antibody response after two doses of either vaccine. However, they did after a third booster shot.

Booster shot benefits

Around a quarter of people receiving two doses of these vaccines may not know if they have enough Nabs. We know that mixing and matching vaccines is safe, so this study provides evidence of the benefit of a booster shot, especially for those in whom two doses may have been insufficient.


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