• The U.S. has agreed to purchase 66 million doses of Moderna's new vaccine formula that targets omicron in a $1.74 billion deal.
• The Moderna order comes on top of a U.S. agreement to buy 105 million doses of Pfizer's updated shots that target omicron for $3.2 billion.
• Many public health officials and scientists believe that updating the vaccines to target omicron BA.4 and BA.5 will provide more durable protection against infection this fall.

The Health and Human Services Department has secured 171 million doses of new Covid vaccines that target the omicron variant in preparation for a fall booster campaign.

The U.S. has agreed to purchase 66 million doses of Moderna's new vaccine formula in a $1.74 billion deal, HHS announced Friday. The Moderna order comes on top of a $3.2 billion agreement to purchase 105 million doses of Pfizer's updated shots that target omicron.

HHS expects to receive the first deliveries from Pfizer and Moderna early this fall. The new vaccines that target omicron still need to be authorized by the Food and Drug Administration and cleared by the Centers for Disease Control and Prevention.

The FDA last month told Pfizer and Moderna to develop vaccines with a new formula that targets the omicron BA.4 and BA.5 subvariants in addition to the original strain of Covid that first emerged in Wuhan, China, in 2019.

Omicron BA.4 and BA.5, the most contagious Covid variants yet, have caused a wave of summer infection that has led to an increase in hospitalizations. Public health officials and scientists are worried that the U.S. faces an even larger wave of infection in the fall as immunity from the original vaccines wanes and people head indoors, where the virus spreads easier, to escape the cold.

Omicron BA.5 makes up nearly 82% of new infections in the U.S. while BA.4 is causing about 13% of new cases. The U.S. is reporting at least 126,000 new infections a day on average as of Wednesday, according to data from the CDC. But the real number of new infections is likely far higher because so many people are using at-home Covid tests, which aren't picked up by the official data.

More than 6,300 people are hospitalized with Covid a day on average as of Tuesday, according to CDC data. More than 360 people are still dying from the virus a day on average as of Wednesday, according to the data.

Moderna's and Pfizer's current shots are based solely on the original strain of Covid that emerged from China more than two years ago. The virus has mutated dramatically since then and, as a consequence, the vaccines are no longer effective at blocking infections. President Joe Biden recently caught Covid, likely BA.5, despite being fully vaccinated and twice boosted. But the current vaccines are still effective at preventing severe disease and death. The president recovered in a matter of days after also taking Paxlovid, Pfizer's oral antiviral treatment.

Many public health officials and scientists believe that updating the vaccines to target omicron BA.4 and BA.5 will provide more durable protection against infection this fall. But the virus is evolving quickly and it's unclear which variant will be dominant by that time.

Congress has failed to pass additional funding to fight Covid. As a consequence, the White House shifted $10 billion from other parts of the pandemic response to secure the new vaccines for the fall. This means that Covid testing, for example, might be weak heading into an expected fall wave.

The 171 million vaccine doses secured by the U.S. are also not enough to cover everyone in the country. Biden administration health officials have warned that the U.S. might have to ration the shots to people at the highest risk of severe disease in the fall.

The U.S. does have the option to purchase another 600 million doses total from Pfizer and Moderna but this will require additional funding from Congress, according HHS.