Six months after receiving the second dose of the two-shot vaccine from Pfizer Inc (PFE.N) and BioNTech SE, many recipients no longer have vaccine-induced antibodies that can immediately neutralize worrisome variants of the coronavirus, a new study suggests.

Researchers analyzed blood samples from 46 healthy, mostly young or middle-aged adults after receipt of the two doses and again six months after the second dose. "Our study shows vaccination with the Pfizer-BioNtech vaccine induces high levels of neutralizing antibodies against the original vaccine strain, but these levels drop by nearly 10-fold by seven months" after the initial dose, Bali Pulendran of Stanford University and Mehul Suthar of Emory University said by email.

In roughly half of all subjects, neutralizing antibodies that can block infection against coronavirus variants such as Delta, Beta, and Mu were undetectable at six months after the second dose, their team reported on Thursday on bioRxiv ahead of peer review.

Neutralizing antibodies are not the immune system's only defense against the virus. Still, they "are critically important in protecting against SARS-CoV-2 infection," said Pulendran and Suthar. "These findings suggest that administering a booster dose at around 6 to 7 months following the initial immunization will likely enhance protection against SARS-CoV-2 and its variants."