Rising coronavirus cases in Israel, where most residents are inoculated with the Pfizer/BioNTech vaccine, offer "a preliminary signal" the vaccine may be less effective preventing mild illness from the Delta variant, a top expert said Monday.
But Ran Balicer, chairman of Israel's national expert panel on Covid-19, stressed it was "too early to precisely assess vaccine effectiveness against the variant" first identified in India in April that is surging across the globe.
That is partly due to the overall low number of cases among fully vaccinated Israelis and because those cases are not evenly distributed across the population, further complicating efforts to reach conclusions about the data.
Balicer, also the chief innovation officer at Clalit, Israel's largest health maintenance organisation (HMO), told AFP that the Delta variant's emergence as the "dominant strain" in the country has led to a "massive shift in the transmission dynamic".
Israel's vaccine rollout that began in December was one of the world's fastest, making IJsrael a closely-watched case study on whether mass inoculation offers a path out of the pandemic.
Vaccinations had brought transmission down to about five local new cases per day, but that figure has risen to around 300 in recent days, with the Delta variant raging.
About half of the daily cases are among children, and half are among mostly vaccinated adults.
"To some extent that could be expected since 85 percent of Israeli adults are vaccinated," Balicer said.
"But the rates in which we see these breakthrough cases make some believe they extend beyond that expected point and suggest some decrease in vaccine effectiveness against mild illness -- but not severe illness -- is likely."
The number of severe cases among vaccinated Israelis has risen in recent days from roughly one every two days up to five cases per day, Balicer said.
He said it was also too early to draw conclusions about the vaccine's effectiveness against serious illness caused by the Delta variant.
But, he added, experts "remain hopeful that the vaccine effectiveness against serious illness will remain as high as it was for the alpha strain" identified for the first time in Britain in December.
Israeli Prime Minister Naftali Bennett warned Sunday, ahead of a weekly cabinet meeting, that "with the Delta variant running amok," Israel may have to reintroduce certain restrictions that were lifted last month to curb transmission.
Balicer said it was unlikely Israel could contain rising cases without further restrictions being reimposed, but voiced hope they would be mild and that Israel's "vaccine wall" of inoculated citizens will help reduce further spread.