4th Covid shot provides little benefit against Omicron: study

The research assessed 270 health care workers at the Sheba Medical Center in Tel Aviv who received a second booster shot of either the Pfizer or Moderna vaccines, four months after initially getting three Pfizer shots.

Each was age-matched against two participants in a control group that had received only three doses of Pfizer, with the median ages in the groups ranging from mid-fifties to early sixties.

Overall, the fourth shot was found to be safe, and topped up recipients' neutralizing antibodies -- which block the coronavirus from infecting cells -- to levels comparable to just after the third dose, prior to when antibody levels started waning with time.

The study took place when spread of the Omicron variant was rife in Israel, and it found that the fourth dose didn't confer much added protection against infection, despite the reduction over time of antibodies among people who'd received just three shots.

Those who got a fourth shot of Pfizer were 30 percent less likely to acquire infection than those who got three doses, while those who got Moderna were 18 percent less likely to get infected.

The study included only a small number of participants and was not a randomized trial -- meaning its conclusions should be interpreted with caution --  but the results point towards only minor benefits from the extra dose.

Protection against symptomatic infection was slightly better in under-40 adult recipients of a fourth jab -- 43 percent for Pfizer and 31 percent for Moderna -- compared to people who received three shots.

The study authors, led by Gili Regev-Yochay, wrote that their research seems to indicate the advantages of three doses of vaccine designed against the original strain of coronavirus had hit a ceiling in terms of immune response, with additional boosters only restoring waned immunity, rather than taking it to new heights.

Outside experts said the findings showed the need to develop new vaccines.