A number of studies are showing that Rockland-based Pfizer's antiviral pill, Paxlovid, is effective against the Omicron variant.

Dr. Supriya Sharma, chief medical adviser with Health Canada, says clinical trials showed treatment with Paxlovid reduced the risk of hospitalization and death caused by COVID-19 by 89% when the medications were started within three days of the beginning of symptoms, and by 85% when started within five days.

Health Canada authorized Paxlovid for adult patients with mild or moderate COVID-19 who are also at high risk of becoming more seriously ill. Health Canada did not authorize it for use on teenagers or on patients who are already hospitalized because of COVID-19.

A study by the second largest health care provider in Israel says that more than 90% of coronavirus patients treated with Paxlovid significantly improved within three days. Maccabi Healthcare says 60% started to feel better within the first day.

Meanwhile, 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 last year. But it said the increased antibodies did not prevent the spread of Omicron.

Omicron is already dominant in many countries and can also infect those who have been vaccinated or had previously been infected by prior versions of the virus. Early studies, however, show it is less likely to cause severe illness than the previous Delta variant. Vaccination and a booster still offer strong protection from serious illness, hospitalization and death.

AP wires were used in this report