Vaccine protection waned this spring but boosters helped, the C.D.C. reports.

Scientists at the Centers for Disease Control and Prevention reported on Friday that the emergence of highly infectious Omicron subvariants in the United States this spring appeared to undermine the protection offered by COVID hospitalization vaccines.

But the first and second booster shots helped increase people’s safety, the agency found. The additional shots raised people’s levels of protection against those Omicron subvariants and restored some of the protection that had been lost as time passed since their last shots.

“When individuals become eligible a booster dose should be received immediately,” the CDC scientists wrote.

The findings, however, came with a notable caveat: Measurements of vaccine effectiveness have been complicated by the number of people who have been infected with the virus, particularly during the wintertime surge of Omicron cases.

They offer some protection from COVID to people with earlier infections. As a result, in studies such as the CDC, which compare virus outcomes in vaccinated and unvaccinated people, vaccines appear to be less protective than they actually are.

CDC scientists used what little was known about patients’ infection histories to try to account for those difficulties. Using data from hospitals in 10 states, the agency’s scientists studied nearly 58,000 hospitals with diagnoses of a Covid-like illness from mid-December to mid-June. The study focused on adults with healthy immune systems.

By the end of April, Omicron’s subvariants known as BA.2 and BA.2.12.1 had overtaken the Omicron variant that had spread across the country in winter.

The study found that after those subvariants took effect, Pfizer-BioNTech and Moderna Vaccines were less effective in preventing people from being hospitalized with COVID than they were during the omicron wave of winter.

Two doses of the vaccines were 24 percent effective against hospitalization after the variant was acquired, compared to 61 percent during the period when Omron’s original version dominated. (These figures are for people who were given a second dose at least five months before.)

This decline probably resulted partly as a result of the subvariant’s ability to evade the immune protection of people from the vaccine, and partly from non-vaccinated people receiving some protection from earlier infections.

A booster dose helped tremendously, even though the benefits of those additional doses waned over time. Once the subvariants became the main sources of infection, a third dose of Pfizer or Moderna increased the vaccine’s effectiveness against hospitalization by 69 percent initially and by 52 percent after four months or more.

The third dose was even more protective during the winter boom of the original version of Omicron.

The CDC found that second boosters were authorized in late March for people age 50 and older with healthy immune systems, and those additional boosters appeared to help people weather the subvariant boom.

The agency said that at least a week after the fourth dose, the vaccine was 80 percent effective against hospitalizations with Covid. This was a substantial increase from the 55 percent effectiveness given by three doses after four months in that age group.

It was not clear how quickly the protection given by that fourth dose would wear off. The study also did not measure the performance of vaccines against the latest omicron subvariant BA.5, which is driving a new surge of cases and hospitalizations. This subvariant has become dominant in new US cases and Omicron appears to be the most evolved form to spread in the country.

With hospitalizations increasing, federal health officials are urging eligible people to receive a booster dose as soon as possible, adding that those shots should help people receive an additional dose of an updated, variant-specific vaccine in the fall or winter. Won’t stop. The agency said the latest results reinforced the need for booster shots.

The CDC said Friday, “Given the recent increase in deaths and hospitalizations associated with the BA.5 variant, everyone should stay up to date with recommended COVID-19 vaccinations, including additional booster doses for those who have are moderate to severely immunocompromised and adults over 50.”