TEL AVIV—About half of adults infected in an outbreak of the Delta variant of Covid-19 in Israel were fully inoculated with the Pfizer Inc. vaccine, prompting the government to reimpose an indoor mask requirement and other measures to contain the highly transmissible strain.
Preliminary findings by Israeli health officials suggest about 90% of new infections were likely caused by the Delta variant, according to Ran Balicer, who leads an expert advisory panel on Covid-19 for the government. Children under 16, most of whom haven’t been vaccinated, accounted for about half of those infected, he said.
Israel is now reassessing its Covid-19 regulations after moving to open up its society and economy following multiple lockdowns last year.
“The entrance of the Delta variant has changed the transmission dynamics,” said Prof. Balicer, who is also the chief innovation officer for Israel’s largest health-management organization, Clalit.
The Delta variant, which first emerged in India in late 2020 and is also known as B.1.617.2, has now been detected in more than 70 countries. In the U.S., public-health experts expect it to soon become the dominant strain.
These so-called breakthrough cases—defined as positive Covid-19 test results received at least two weeks after patients receive their final vaccine dose—are broadly expected as the Pfizer vaccine is highly effective but not 100% foolproof, according to Mr. Balicer.
Israeli health officials are optimistic that even if the variant does spread, evidence from countries such as the U.K. indicate the vaccine will prevent a large increase in severe illness and hospitalizations that plagued the country’s health system in previous outbreaks. Israel has only recorded five severe cases in the past 10 days, Prof. Balicer said, but whether more will emerge is too early to tell.
The number of cases in Israel is relatively low by global standards. New cases of Covid-19 rose to over 200 on Thursday from around 10 a day for most of June.
Those exempt from the mask requirement included children under seven, people with disabilities that prevent them from wearing a mask or two workers who work regularly together. The government had canceled the indoor mask requirement 10 days ago and dropped most other preventive measures after running one of the world’s fastest vaccination campaigns. About 80% of Israelis of age 16 and above have received two doses of the vaccine that was developed by Pfizer and German partner BioNTech SE.
Israel was an early test case for the effectiveness of the vaccine after outbreaks last year at one point gave it one of the world’s highest per-capita infection rates. Since the start of the pandemic, 840,522 of the country’s 9.3 millions citizens have been infected, of which 6,429 died.
Israeli health officials said the highly contagious strain had likely entered the country through its main international airport, near Tel Aviv, where a system meant to vet every new arrival through testing was overloaded in recent days amid a surge in foreign travel.
On Wednesday, the government delayed allowing foreign nationals to enter into the country for tourism from July 1 to Aug. 1 and reimposed a mask requirement inside airports.
“Our goal at the moment, first and foremost, is to safeguard the citizens of Israel from the Delta variant that is running amok in the world,” Israeli Prime Minister Naftali Bennett said Wednesday.
Corrections & Amplifications
About half of adults infected in an outbreak of the Delta variant of Covid-19 in Israel were fully inoculated with the Pfizer vaccine, according to a clarification by Prof. Balicer of an earlier statement. An earlier version of this article incorrectly said about half of people infected in the outbreak were fully inoculated. (Corrected on June 25)
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