A fourth dose of COVID-19 vaccine boosts antibodies to higher levels than the third jab but provides only partial protection against the Omicron variant of coronavirus, according to a preliminary study conducted in Israel.

The yet-to-be peer-reviewed study tested the efficacy and safety of vaccines and analysed whether the combination of vaccines from different manufacturers would affect the rate of increase of antibodies. The researchers from Sheba Medical Center in Israel gave second booster shots in a trial to its staff, and studied the effect of the Pfizer booster in 154 people after two weeks and the Moderna booster in 120 people after one week.

Preliminary results indicate that one week after the administration of a fourth dose of Moderna vaccine, the rate of antibodies increased similarly to that found one week after the administration of a fourth dose of Pfizer. The study also found that two weeks after the administration of a fourth dose of Pfizer, there was a further increase in antibodies, slightly more than that measured after the first week. Additional results show that the safety level of the vaccine is similar in both in the Pfizer and Moderna vaccines.

"The increase observed in the level of antibodies is slightly higher than the peak level observed after the booster dose was given -- the third dose, said Professor Gilli Regev-Yochai from Sheba Medical Center. "We understand that despite the significant increase in antibodies after the fourth vaccine, this protection is only partially effective against the Omicron strain, which is relatively resistant to the vaccine," Regev-Yochai said.

The researchers noted that the vaccines, which were very effective against the previous strains, is less effective against the Omicron variant. The fourth dose restores and even slightly exceeds the protection provided by the third dose, they said. "Although the vaccines we have today do not provide optimal protection against infection with the Omicron strain, it is correct to continue the vaccination campaign for at-risk populations," Regev-Yochai added.