President Joe Biden said Thursday the US had signed deals to acquire 200 million more COVID-19 vaccine doses as officials look to immunize the vast majority of the population by July.

The deals with Pfizer and Moderna, which the administration had announced it was seeking last month, increase the country’s total vaccine supply by 50%, to 600 million doses.

“Just this afternoon, we signed … final contracts for 100 million more Moderna and 100 million more Pfizer vaccines,” Biden said after touring the National Institutes of Health near Washington.

“We’re now on track to have enough supply for 300 million Americans by the end of July,” he added. Both vaccines currently authorized for emergency use in the US require two shots.

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The US immunization campaign got off to a shaky start in December but has since improved: 46.4 million shots have now reached arms, and at least 34.7 million people have received one or more doses, roughly 10% of the population.

The vaccine drive entered a new phase Thursday as 6,500 pharmacies began scheduling shots, but a wave of optimism brought about by falling cases could soon be tested by dangerous variants.

A million doses were being shipped to drugstores and supermarket pharmacies nationwide, with chains announcing they would begin administering the first shots Friday.

The pharmacy program will eventually expand to 40,000 outlets, while the federal government has also used emergency legislation to ramp up vaccine production, opened mass vaccine sites in stadiums, and has begun a separate program to reach underserved communities.

Anthony Fauci, the president’s chief advisor on the pandemic, told NBC that by April, vaccines would start being available to anyone who wanted them.

While the shots are currently only available to adults, officials have voiced hope that ongoing studies would allow them to be approved for children in the coming months.

Since peaking on January 8 following the holiday season, the US outbreak has been in steep decline, with an average of just over 100,000 daily cases — down from more than 300,000.

Hospitalizations and fatalities are following suit — but the worst-hit country in the world is still expected to reach the latest grim milestone of half a million deaths this month.

And, because of the fragmented nature of the US health system, widespread confusion persists among the public over where and when they can get vaccinated.

Stark racial and economic disparities have also emerged. In the capital Washington, vaccines have been disproportionately allocated to residents of the city’s affluent, white neighbourhoods where people have been quicker to register by phone or online.

Between the number of people already vaccinated and the number of people who’ve already been infected, it’s thought that around 40% of the population might have some immunity, which is also contributing to the slowdown in new cases.

But while the numbers are headed in the right direction, experts warn it’s not time to get complacent.

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The winter surge was compounded by people gathering over the holidays for Thanksgiving, Christmas and the New Year, and if those patterns were repeated during last Sunday’s Super Bowl, we can expect to see a spike.

The highly-transmissible B.1.1.7 variant that was first identified in Britain is also rapidly spreading in the US and is set to become the dominant strain by March, according to multiple studies.

This has the potential to reverse current progress, just as it did for example in Ireland, which had gotten its outbreak under control by late 2020, but was hit by a B.1.1.7 driven exponential wave in January and still recovering.

A smaller amount of this variant can cause COVID-19, so attention has shifted toward better masking practices.

The Centers for Disease Control and Prevention confirmed Wednesday that wearing a cloth mask on top of a medical mask, or knotting and tucking a medical mask to fit more snugly, offers significantly better protection against catching or spreading COVID-19.