A second study from Qatar looked at actual infections among the highly vaccinated population of that small Gulf nation. People there mostly got Pfizer/BioNTech’s vaccine, also known as BNT162b2.

“BNT162b2-induced protection against infection builds rapidly after the first dose, peaks in the first month after the second dose, and then gradually wanes in subsequent months,” Laith Abu-Raddad of Weill Cornell Medicine-Qatar and colleagues wrote. “The waning appears to accelerate after the fourth month, to reach a low level of approximately 20% in subsequent months,” they added.

Nonetheless, protection against hospitalization and death stayed at above 90%, they said.

The waning protection may involve behavior, they noted. “Vaccinated persons presumably have a higher rate of social contact than unvaccinated persons and may also have lower adherence to safety measures,” they wrote. “This behavior could reduce real-world effectiveness of the vaccine as compared with its biologic effectiveness, possibly explaining the waning of protection.”