WASHINGTON (AP) — The campaign to vaccinate elementary school age children in the U.S. is off to a strong start, health officials said Wednesday. But experts say there are signs that it will be difficult to sustain the initial momentum. About 900,000 kids aged 5 to 11 will have received their first dose of the COVID-19 vaccine in their first week of eligibility, the White House said. Nearly 20,000 pharmacies, clinics and physicians’ offices are offering the doses. An initial surge of demand was expected, as many parents arranged for their kids to get vaccinated before the holidays. But that demand could wane, as it did for adolescents earlier in the year.