Both Pfizer and Moderna seemed like equally protective vaccines when they were first released, boasting nearly the same high efficacy rates and similar mRNA technology. But as the months have passed, the two vaccines have diverged somewhat in their real-world effectiveness. In August, Mayo Clinic researchers found that the Moderna vaccine was able to shield more against breakthrough infections, proving to be 76 percent effective compared to Pfizer’s 42 percent. Then in September, data from the Centers for Disease Control and Prevention (CDC) found that Moderna’s protection stayed consistent, while Pfizer’s effectiveness dropped four months after vaccination. Now, new research seems to show that the Pfizer vaccine’s protection plummets even sooner than previously believed.
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Two studies published in the New England Journal of Medicine on Oct. 6 show that the immune response from Pfizer’s vaccine begins to drop off as soon as two months after the second dose. One of the studies from Israel analyzed more than 4,800 health care workers over the course of six months, who were tested monthly for antibodies from Dec. 19 to July 9. According to this study, the highest antibody titers were observed through the first month after Pfizer’s second dose, but both IgG and neutralizing antibodies started reducing significantly each month after. Recipients’ IgG antibodies decreased at a steady pace two to six months after the second shot, while neutralizing antibodies plummeted in the second and third months before tapering off to a steady decline.
“The decrease in neutralizing antibody titers was brisk initially, in the period of up to 70 to 80 days, but slowed thereafter,” the study noted.
The other study from Qatar looked at COVID infections among more than 900,000 residents fully vaccinated with Pfizer between Dec. 21 and Sept. 5. According to the study, the vaccine’s effectiveness against infection peaked at 77.5 percent in the first month after the second dose. But by month two, the effectiveness of Pfizer’s vaccine had started to decline gradually. “The effectiveness reached a low level of approximately 20 percent in months five through seven after the second dose,” the researchers noted in their study.
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Pfizer has long been citing studies that show that its protection wanes over time, which is why it has pushed for booster shots. The U.S. Food and Drug Administration (FDA) and CDC recently authorized an additional dose of the Pfizer vaccine, but only for select groups. This includes anyone who got their second dose at least six months ago and is 65 years or older, lives in long-term care settings, has underlying medical conditions, or lives or works in high-risk settings.
But the researchers of the two studies suggest that boosters should not be restricted. “I would be more than surprised if we are not going to start to see a lot of breakthrough infections in the United States” among those who only got two doses, Gili Regev-Yochay, MD, one of the authors of the Israeli study, told Bloomberg. The researchers for the Qatar study said that their findings “suggest that a large proportion of the vaccinated population could lose its protection against infection in the coming months, perhaps increasing the potential for new epidemic waves.”
The researchers for the Qatar study did note that the decline in protection against infection might not be solely related to the vaccine’s immune response, however. “Vaccinated persons presumably have a higher rate of social contact than unvaccinated persons and may also have lower adherence to safety measures. This behavior could reduce real-world effectiveness of the vaccine as compared with its biologic effectiveness, possibly explaining the waning of protection,” they said.
The Qatar study also found that Pfizer remained consistently effective against COVID-related hospitalization and death. The researchers reported a 96 percent effectiveness against severe COVID for the Pfizer vaccine in the first two months after the second dose, which did not “did not decline over time, except possibly in the seventh month after the second dose when there was a hint of a decline, but the case numbers were small.”
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