Israel is making preparations to ensure it has a sufficient vaccine supply in case a fourth round of COVID-19 shots is needed, the country’s top health official said on Sunday.

“We don’t know when it will happen; I hope very much that it won’t be within six months, like this time, and that the third dose will last for longer,” Health Ministry Director General Nachman Ash said in an interview with Radio 103FM.

Israel began a drive to administer booster shots at the beginning of August, and has so far inoculated about 2.8 million people with a third dose of coronavirus vaccine.