Israeli health officials are returning to mask mandates at indoor facilities following a small spate of positive COVID Delta variant cases in adults who had been fully vaccinated with Pfizer's coronavirus vaccine. That is, these adults had received two doses of the vaccine, and it had been two weeks since their second dose, the generally accepted timeline for full vaccination.

The Delta variant has rapidly become the main source of new COVID cases in certain parts of the globe including India and the U.K. It also makes up an increasing share of new cases in the U.S., clustered in states with low vaccination rates. The mutation, originally identified in India, appears to be far more transmissible than the novel coronavirus and could be a particular threat to younger people.

In Israel, expert advisers to the government estimate that about 90% of new cases are linked to the Delta variant, according to the Wall Street Journal. About half of the Delta variant cases examined by experts occurred in children under the age of 16 who haven’t been vaccinated. But about half of adults who were infected in this outbreak were fully immunized with the COVID vaccine.

That doesn’t mean it’s time to panic quite yet. Early studies in the U.K., where the Delta variant is also prominent, have suggested that the Pfizer vaccine in fully immunized people is 96% effective at preventing hospitalizations for those who become infected with the Delta variant. And in the past five days, Israel has recorded only five severe cases of COVID, according to Ran Balicer, a COVID adviser to the Israeli government. Of course, illness and hospitalizations (and deaths) are all lagging indicators. It will take more time to tell just how effective Pfizer's vaccine is at preventing debilitating COVID illness in those with the Delta variant, or whether booster shots will be required to address it.

Pfizer is sounding a confident note in its vaccine's efficacy against the strain. “The data we have today, accumulating from research we are conducting at the lab and including data from those places where the Indian variant, Delta, has replaced the British variant as the common variant, point to our vaccine being very effective, around 90%, in preventing the coronavirus disease, COVID-19,” Pfizer’s Israel medical director Alon Rappaport told a local Israeli station, according to Reuters.

In short：It’s going to take at least a few more months to get a better grasp of the relationship between available COVID vaccines and the Delta variant.