The United States has reached a deal to buy 200 million more doses of Moderna’s Covid-19 vaccine, with the option to purchase variant-specific boosters that are in the pipeline, the biotech company said Wednesday. The purchase brings Washington’s confirmed order to 500 million doses of the two-shot regimen. Moderna has so far supplied 217 million doses, with 110 million more to be delivered by the end of this year and 90 million in the first quarter of 2022. “We appreciate the collaboration with the US government for these additional doses of the Moderna Covid-19 vaccine, which could be used for primary vaccination, including of children, or possibly as a booster if that becomes necessary to continue to defeat the pandemic,” said the company’s CEO Stephane Bancel. “We remain focused on being proactive as the virus evolves by leveraging the flexibility of our mRNA platform to stay ahead of emerging variants.” The US crossed 600,000 Covid deaths on Tuesday, but the rate of new cases is now at its lowest since the pandemic began last spring, thanks in large part to a strong vaccination campaign that has so far covered 65 percent of adults with at least one dose. But vaccinations hit a highwater mark in April and have been declining since, and experts fear under-immunized areas in the country’s South could incubate new waves. The next big threat is likely to come from the Delta variant, which was first identified in India and currently dominant in Britain, where it has driven a new rise in cases and delayed reopening measures. British authorities have reported Delta is 60 percent more transmissible than Alpha, the variant first identified in Britain. Vaccine effectiveness for the Delta variant is comparable to that for the Alpha strain after two doses, but appears to be significantly lower after just one dose.