Pfizer says third vaccine shot 'strongly' boosts immune response against delta variant

Pfizer said Wednesday that a third dose of its COVID-19 vaccine "strongly" boosts the immune response against the delta variant.

The company posted slides ahead of an earnings call, with data showing that antibody levels were five times higher after a third dose among people aged 18 to 55 against the delta variant, and 11 times higher among 65- to 85-year-olds.

Pfizer made waves earlier this month when it said it would be applying for Food and Drug Administration (FDA) authorization for a third dose of its vaccine.

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Still, it is not fully clear yet when and if a booster dose will be needed. It is possible booster shots will only be needed for more vulnerable people like the elderly or those with compromised immune systems.

Experts also point out that antibody levels are not the only measure of protection and that there are other parts of the immune system that are activated against the coronavirus too. Some have noted Pfizer also has a financial incentive to sell more doses of its vaccine to use as boosters.

The Centers for Disease Control and Prevention and the FDA pushed back in a rare joint statement earlier this month, saying booster shots were not currently needed. But federal health officials are also leaving open the possibility that they could be needed at some point, at least in certain populations.

Data from the United Kingdom published in the New England Journal of Medicine this month found that the Pfizer vaccine was 88 percent effective against the delta variant with just two doses. The vaccine is even more effective against preventing severe outcomes.