A fourth dose of a coronavirus vaccine offers limited protection against the Omicron variant, an Israeli study has concluded on Monday.
Preliminary results from the study conducted at Sheba Medical Center showed that after receiving the shot, participants developed higher coronavirus antibody levels, which were only partly effective in preventing Omicron infections.
Israel's largest hospital began administering the fourth vaccine dose to 150 staff last month, with that number eventually swelling to 270, in a trial aimed at gauging whether a second booster is necessary nationwide.
Participants were given doses of either Pfizer/BioNTech or Moderna vaccines and compared their coronavirus antibody levels to those of their counterparts who did not receive the second booster.
"The increase observed in antibody level after the administration of the fourth dose of either Moderna or Pfizer is slightly higher than the peak level observed after the administration of the third dose, said study director Prof. Gili Regev-Yochay.
"However, using Sheba's exclusive data on Omicron morbidity among hospital staff participating in serological research, we see that despite the significant increase in antibodies after the fourth shot, this protection is only partially effective against the Omicron variant, which is relatively resistant to the vaccine."
Earlier this month, Israel became the first country in the world to approve the fourth coronavirus jab for people over 60 as well as to health workers in a bid to stave off Omicron's spread
Prof. Regev-Yochay said she believed it was the right call to offer the extra shot to older citizens, but suggested waiting until a vaccine tailored for Omicron becomes available before administering it to the rest of the population.
The findings largely confirm fears Prof. Regev-Yochay voiced talking to Ynet earlier this month that the second booster was ineffective against the strain.
"A five-fold increase [in antibody levels] is good, but it's not enough, it does not give the same effect as the first booster shot. If these results bring us back to antibodies level of approximately four months ago, then it means we will have to get vaccinated every four months, and that's not the goal," said Prof. Regev.