U.S. regulators committed last week to updating the 2020 vaccine recipes for this fall’s COVID-19 booster campaign with new formulas meant to defend against the ultra-contagious omicron subvariants.

The Biden administration is betting that the new formulas, the centerpiece of an effort to drastically speed up vaccine development, might appeal to the half of inoculated Americans who have spurned booster shots, a key constituency in the fight against future COVID-19 waves.

The most evasive forms of omicron yet, known as BA.4 and BA.5, appear to be driving a fresh surge of cases across much of the United States.

COVID-19 deaths in the United States, which had been hovering for months near their lowest levels of the pandemic, are rising again.

“We’re hoping that we can convince people to go get that booster,” said Dr. Peter Marks, who oversees the vaccines office at the Food and Drug Administration, “and help mature their immune response and help prevent another wave.”

Now, the question is whether those modified boosters will arrive in time. In a bid to match the latest forms of the virus, the FDA asked vaccine manufacturers to tailor their new shots to the BA.4 and BA.5 subvariants, rather than to the original version of omicron from last winter.

Virus experts said that a subvariant vaccine would generate not only the strongest immune defenses against current versions of the virus but also the type of broad antibody response that will help protect against whatever form of the virus emerges in the months ahead.

But building a fall booster campaign around vaccines at the forefront of the virus’s evolution could also come at a cost. Pfizer and Moderna said that they could deliver subvariant vaccine doses no earlier than October. .

In contrast, a vaccine targeting the original version of omicron is closer at hand: Moderna and Pfizer have already started making doses tailored to the original form of omicron, and Moderna said it could start supplying them this summer. Whether the benefits of a newer subvariant vaccine outweigh the drawbacks of having to wait longer depends on when exactly it arrives and how much havoc the virus wreaks before then, scientists said.