Oct. 8, 2021 -- The protection from Pfizer’s COVID-19 vaccine may begin to wane against infection after 2 months, but it still prevents hospitalization and death for at least 6 months, according to two new studies published Wednesday in TheNew England Journal of Medicine.

The new findings support what Pfizer, the CDC, and top health officials have said in recent weeks: Initial protection against getting the coronavirus itself may drop over time, but people who get the vaccine avoid the worst effects of severe disease.

In the first study, researchers in Qatar estimated vaccine efficacy against COVID-19 infection and severe, critical, or fatal cases between Jan. 1 and Sept. 5. They based the report on infections among 900,000 vaccinated people in Qatar.

After the first Pfizer dose, protection against infection was “negligible” for the first 2 weeks but jumped to about 37% in the third week. Protection then reached a peak of about 78% in the first month after the second dose.

Efficacy fell gradually after that, with the decline speeding up after the fourth month. For some people who got the vaccine, protection dropped as low as 20% between 5 and 7 months after the second dose.

“These 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 wrote.

The effectiveness against symptomatic infection was higher than the effectiveness against asymptomatic infection, but both rates waned over time, the researchers wrote. A similar pattern was seen for the different variants, including the Beta and Delta variants.

Effectiveness against severe, critical, and fatal COVID-19 cases increased to 66% by the third week after the first dose and reached 96% in the first 2 months after the second dose. The protection lasted at this level for about 6 months.

“Protection against SARS-CoV-2 infection appeared to wane rapidly following its peak after the second dose, but protection against hospitalization and death persisted at a robust level for six months after the second dose,” the researchers wrote.

In the second study, researchers in Israel looked at COVID-19 antibodies among more than 4,800 health care workers who were tested regularly after vaccination. Among those, 20 had breakthrough infections.

The research team found that the level of antibodies circulating in the blood decreased at a consistent rate over 6 months. The level of neutralizing antibodies, which correlates with protection, decreased rapidly in the first 3 months but then slowly declined after that.

Six months after the second dose, neutralizing antibodies were substantially lower among men than women. They were also lower for people ages 65 and older and those with weak immune systems.

Neutralizing antibodies were significantly higher in those who had a body mass index of 30 or above. But the researchers said it’s unclear whether the higher level is more protective and whether vaccinated obese people face higher or lower risks for breakthrough infections.

“As this pandemic continues to evolve, the importance of determining immune correlates of protection after vaccination becomes clearer,” the researchers wrote. “Strategies to prolong host immunity need to be evaluated in order to protect the population against SARS-CoV-2 and its variants.”

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The New England Journal of Medicine: “Waning of BNT162b2 Vaccine Protection against SARS-CoV-2 Infection in Qatar,” “Waning Immune Humoral Response to BNT162b2 Covid-19 Vaccine over 6 Months.”

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