Health workers who received a fourth shot of Pfizer Inc.'s messenger RNA vaccine were considerably less likely to contract COVID than their triple-vaccinated peers, according to a study.

According to Bloomberg, the findings, published Tuesday in the open access journal of the American Medical Association, are the latest to indicate the benefits of a second booster against Omicron breakthrough infections. The study's authors suggested that an extra dose may be used to minimise medical staff shortages and save health-care systems during times of crisis.

The study was carried out in Israel, where a rapid vaccine roll-out provided researchers with real-world data on vaccine efficacy. In January, the country began providing a second booster to the elderly, health workers, and individuals with weakened immune systems.

With the emergence of the BA.5 Omicron variation, the US is now evaluating whether to increase eligibility for second booster shots.

Doctors, nurses, and other health-care workers who had a fourth mRNA shot in January had a 7% chance of developing a new infection. Those who received three doses, the third of which was provided by the end of September, had a 20% infection rate.

According to the experts, many Israeli health personnel skipped the fourth dose in January, figuring it wouldn't make much of a difference.

“The common assumption was that the combination of reduced virulence of the Omicron variant and the protection given by the first three vaccine doses created no added value for the fourth vaccine,” they wrote. But for medical staff, they argued such a difference matters because “quarantine and isolation of a large number of health-care workers may impair the ability of the health system to function."