President Joe Biden on Wednesday directed his administration to procure 100 million more doses of the single-shot Johnson & Johnson coronavirus vaccine to boost US supply in the event of "unexpected challenges" in the pandemic. The White House order comes as it seeks to speed up the process of inoculating people to curb the spread of Covid-19, which has killed more than 528,000 people in the US alone. Mr Biden highlighted the move at a meeting with the chief executives of Johnson & Johnson and Merck, which has agreed to partner with Johnson & Johnson to produce its vaccine. "We need maximum flexibility. There's always a chance that we'll encounter unexpected challenges," Mr Biden said. "A lot can happen. A lot can change. And we need to be prepared." The president has said previously that the US will have enough supplies to vaccinate the entire population by the end of May. But the White House is eager to have more on hand. If the country had additional supply, it would be shared, Mr Biden said. "If we have a surplus, we're going to share it with the rest of the world," he said. The US has already paid over $1 billion for 100 million doses from Johnson & Johnson and had an option to purchase an additional 200 million doses. As of Tuesday morning, about 128 million doses of coronavirus vaccines from the three authorised providers had been distributed in the US and about 96 million shots had been administered, according to the US Centres for Disease Control and Prevention. But Johnson & Johnson’s manufacturing has been slower than hoped, and it was not expected to deliver any vaccine doses this week. It is expected to resume shipments later in March. A Johnson & Johnson spokesman, Jake Sargent, said the company was on track to deliver the first 100 million doses of the vaccine ordered by the US, but the company has come up short on its shipments this month and expects to deliver only 20 million of the 37 million doses it had originally promised to have ready by now. Merck, meanwhile, will help with the final manufacturing steps called "fill and finish" – expected to be up and running in May – and production of the vaccine itself. Johnson & Johnson, which has committed to producing 1 billion doses this year, also said it is continuing to seek more manufacturing partners. Centres for Disease Control Director Rochelle Walensky said the recent plateau of Covid-19 cases appeared to be trending downward, while new hospital admissions and coronavirus-related deaths were also decreasing. "While these trends are starting to head in the right direction, the number of cases, hospitalisations and deaths still remain too high and are sombre reminders that we must remain vigilant as we work to scale up our vaccination efforts across this country," she said. The US approved the Pfizer-BioNTech and Moderna vaccines in December, and those companies have been working to increase their production since then. Pfizer and Moderna have committed to supplying the country with 120 million doses and 100 million doses, respectively, by the end of March. They have agreed to supply 300 million doses each by the end of July. "We want to be oversupplied and overprepared," White House spokeswoman Jen Psaki said earlier on Wednesday.