Two recent studies confirm that the immunological protection provided by two doses of Pfizer's Covid-19 vaccine wears off after about two months, while protection against severe illness, hospitalisation, and death remains strong.
Pfizer has asserted earlier that after a few months, protection from the first two doses of their vaccine begins to run off, reports CNN.
Pfizer received approval from the US Food and Drug Administration last month for booster doses of its vaccine to be given six months after the initial two doses have been completed.
The studies, which were published in the New England Journal of Medicine and come from Israel and Qatar, back up claims that even fully vaccinated persons should take precautions against infection.
One study from Israel covered 4,800 health care workers and showed antibody levels drop off rapidly after two doses of vaccine "especially among men, among persons 65 years of age or older, and among persons with immunosuppression."
"We conducted this prospective longitudinal cohort study involving health care workers at Sheba Medical Center, a large tertiary medical center in Israel," Sheba's Dr. Gili Regev-Yochay and colleagues wrote.
The researchers observed that levels of so-called neutralising antibodies, which are the immune system's initial line of defense against infection, are linked to protection against infection, but they focused on antibody levels in this study.
Immunity for those who were vaccinated after a natural Covid-19 infection lasts longer, according to the Israeli study.
The vaccine is most effective among those who have recovered from the virus and have then been vaccinated.
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.
However, protection against hospitalisation and death stayed at above 90%, they said.