(Bloomberg) -- Researchers from Israel told a panel of U.S. vaccine experts weighing a potential Covid-19 booster dose from Pfizer Inc. and BioNTech SE on Friday that it’s unclear how long the benefit of such a shot would last.

A big part of the case for booster shots is data from Israel showing that, in the short term, a third dose of the vaccine dramatically lowered infections and severe illness in the short term in people over age 60 and older. 

However, it isn’t yet clear whether the enhanced protection boosters could provide would be short-lived, or if the benefit would be lasting.

Read More: Booster Debate Comes Down to Pivotal FDA Meeting

When asked by members of a Food and Drug Administration advisory panel whether more shots would be needed to keep Covid-related illness away, the Israeli scientists said it was simply too soon to know.

“This is very early, we can’t really tell,” said Sharon Alroy-Preis from the Israel Ministry of Health. “It is not really clear where this is going.”

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