JERUSALEM (AP) — An Israeli hospital on Monday said preliminary research indicates a fourth dose of the coronavirus vaccine provides only limited defense against the omicron variant that is raging around the world.

Sheba Hospital last month began administering a fourth vaccine to more than 270 medical workers — 154 who received a Pfizer-BioNtech vaccine and 120 others who received Moderna's. All had previously been vaccinated three times with the Pfizer-BioNtech vaccine.

The clinical trial found that both groups showed increases in antibodies "slightly higher" than following the third vaccine...