By Felicia Schwartz and Dov Lieber 

TEL AVIV -- Early data from Israel suggests Covid-19 infection rates began to decrease among a group of vaccine recipients two weeks after they received the first shot of Pfizer Inc. and BioNTech SE's vaccine, offering important insights to other countries as they roll out their own campaigns.

The small country -- whose roughly nine million population is about the size of New York City's -- has vaccinated nearly a fourth of its population in just under a month, the first country to hit that mark as it fights an upsurge in new infections.

Israel's largest health-care provider, Clalit Health Services, compared test positivity rates among 200,000 people over 60 who received the vaccine with 200,000 that didn't. Until day 14, there was little difference between the two groups. But after that, the data showed a 33% fall in infection rates among those who had already been vaccinated compared with those who hadn't.

Clalit noted that the number of people infected was statistically significant, but said it wouldn't release final numbers until its study is published.

Pfizer says people must receive both doses of the vaccine for it to be fully effective. In Pfizer's trials, the vaccine was shown to take about 12 days before it started to protect people.

The Clalit study suggests that the first dose could reduce infections among those vaccinated as early as two weeks after injection.

While Pfizer's research was based on people who had symptomatic Covid-19 and a positive lab test, Clalit's data analysis was based on Covid-19 tests provided both to people who felt symptoms and to those who didn't, such as people who came into contact with others who had Covid-19, according to Ran Balicer, chief innovation officer at the Israeli health-care provider.

Israel, like many other countries, has given priority to those over 60 in the first stage of its vaccination drive, making the group the best fit for study, said Mr. Balicer, who is also chairman of Israel's national advisory team on Covid policy. Nearly three out of four people in this age group have received the first dose of the vaccine since Israel's campaign began on Dec. 20.

Israel has committed to providing Pfizer with real-time data about their vaccine, from effectiveness to side effects, which Israeli officials said helped it procure early shipments from the vaccine maker.

Separately, Israel's Health Ministry published data about side effects from the vaccine, saying they were similar in frequency and character to other vaccinations given to its people.

Of the 1.7 million who were vaccinated by the time the report was published, just 1,127, or 0.06%, reported side effects, which included weakness, headaches, dizziness and fever as well as pain or swelling at the site of the shot. So far 15 people needed to be hospitalized after the shot, mostly due to pre-existing conditions.

Even as Israel accelerates the vaccine's rollout, aiming to inoculate the majority of its population by March, its Health Ministry officials warned that people must still be vigilant.

Israel has hit a pandemic peak in recent days at an average of over 9,000 new daily infections and is averaging between 40 and 50 deaths a day. As of Thursday morning, 1,842 patients with coronavirus were hospitalized, of which 1,095 were in critical condition. Israel last month imposed a third lockdown to contain the new surge in infections.

Among those who are critically ill, 17% of them have already received their first doses of the Pfizer vaccine.

Israeli health officials say the latest surge can be explained by the fact that much of the population remains unvaccinated. People in ultra-Orthodox and Arab communities that are hardest hit aren't taking the shots as much as those in other communities and the spread of the virus has increased since the detection of the new U.K. coronavirus variant, health officials say.

Write to Felicia Schwartz at Felicia.Schwartz@wsj.com

 

(END) Dow Jones Newswires

January 14, 2021 16:25 ET (21:25 GMT)

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