Pfizer now hopes to tell U.S. regulators how well its COVID-19 vaccine works in the littlest kids by late May or early June.

Pfizer is testing three extra-small doses of its vaccine in children under 5 after two shots didn't prove quite strong enough. Initial results had been expected last month but the company laid out the latest timeline Tuesday during its discussion of quarterly financial results.

Currently in the U.S., only children ages 5 or older can be vaccinated, using Pfizer’s vaccine -- leaving 18 million younger tots unprotected.

Rival Moderna hopes to be the first to offer vaccinations for the youngest children. Last week, it filed with the Food and Drug Administration data it hopes will prove two of its low-dose shots work in children younger than 5. Moderna also has filed FDA applications for older kids, although the agency hasn’t ruled on them.

The FDA already has set tentative dates in June to publicly review data on COVID-19 vaccines for tots under 5, from either or both companies.

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