A study from Qatar found protection against asymptomatic infection with Pfizer's vaccine declines a few months after the second dose.
Another Pfizer study from Israel showed neutralizing antibodies waning dramatically after 6 months.
But Pfizer's protection against severe disease and death is still well over 90% for at least 6 months.
People who get Pfizer's two-shot vaccine may still catch COVID-19 in the months after they are fully vaccinated - though those infections may be so mild they fly entirely under the radar.
Two new studies, published in the New England Journal of Medicine on Wednesday, show that Pfizer's mRNA shots still remain effective in guarding against hospitalization and death for at least six months, though protection against milder disease as well as antibody levels can fall - or at least they did in the face of the Beta and Delta variants.
The new findings affirm what Pfizer, Moderna, and the Centers for Disease Control and Prevention have indicated in recent weeks - that the mRNA vaccines' ability to protect the body from coronavirus infection may wane over time, meriting a third shot.
Strong protection against the worst things COVID-19 can do persisted for at least half a year
In the first study, researchers in Qatar (a highly inoculated country with more than 82% of people fully vaccinated) investigated more than 900,000 PCR tests of people vaccinated with Pfizer (the most popular shot there), and found that their protection against any infection started to decrease markedly about four months after their second jab.
The researchers found that Pfizer's protection against infection was "negligible" just after a first dose, rising to 36.8% three weeks later. When people then received their second shot, their vaccine protection jumped to 77.5% within about four weeks, and protection against "any severe, critical, or fatal case of COVID-19 increased rapidly," the researchers said, reaching 96% or higher in the first two months that people were fully vaccinated.
That strong protection against the worst things COVID-19 can do to a person persisted for at least half a year.
"No evidence was found for an appreciable waning of protection against hospitalization and death, which remained robust - generally at 90% or higher - for 6 months after the second dose," the researchers said.
'Protection against asymptomatic infection diminished more quickly'
Meanwhile, Pfizer's protection against milder and more negligible COVID-19 infections declined.
After people had been fully vaccinated for about five to seven months, the researchers observed Pfizer's vaccine effectiveness hovering around 20%, though only about a third of those infections were diagnosed "on the basis of symptoms," suggesting that many of them were silent, asymptomatic infections.
"Protection against asymptomatic infection diminished more quickly than that against symptomatic infection, as would be expected in a vaccine that prevents symptoms," the researchers said. "These findings suggest that a large proportion of the vaccinated population could lose its protection against infection in the coming months."
Other research from Qatar suggests that breakthough infections are less infectious than those in unvaccinated individuals, making them less likely to spread.
The second NEJM study, conducted in Israel, looked at 4,868 healthcare workers who'd been fully vaccinated with Pfizer's vaccine. It reported that their neutralizing antibodies to COVID-19 are "substantially" lowered by six months after receiving their second dose of Pfizer's vaccine - a trend that's especially true for men, people over age 65, and people with weakened immune systems.
Even so, only 20 of those healthcare workers had breakthrough infections during the study period, suggesting, again, that vaccine protection remains robust for many months after people get jabbed.
It's normal for neutralizing antibodies to decline after vaccination, and neutralizing antibodies are not the only element of immune response that protects us from reinfection, but vaccines for other conditions such as mumps, measles, and rubella only show small decreases of about 5% to 10% each year in neutralizing antibody levels, the researchers said.
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