The CDC reports that vaccine protection has weakened this spring, but boosters have helped.

Scientists from This was reported by the Centers for Disease Control and Prevention. on Friday.

The agency found that the first and second repeated shots helped bolster people’s defenses. The extra shots increased the men’s defenses against these Omicron sub-variants and restored some of the protection that had been lost over time since their last shots.

“Booster doses should be given immediately when people become eligible,” the CDC scientists wrote.

The conclusions, however, came with a notable caveat: Measuring vaccine effectiveness was complicated by the number of unvaccinated people infected with the virus, especially during the winter spike in Omicron cases.

These previous infections give people some protection from Covid. As a result, in studies such as the US Centers for Disease Control and Prevention, which compare virus outcomes in vaccinated and unvaccinated people, vaccines appear to be less protective than they actually are.

CDC scientists used what little they had about patients’ infection histories to try to explain these difficulties. Using data from hospitals in 10 states, agency scientists studied about 58,000 hospitalizations diagnosed with Covid-like illness from mid-December to mid-June. The study focused on adults with healthy immune systems.

By the end of April, subvariants of Omicron are known. as BA.2 as well as BA.2.12.1 surpassed the version of Omicron that had spread across the country over the winter.

The study found that after these sub-options became dominant, the Pfizer-BioNTech and Moderna vaccines were less effective in preventing Covid people from being hospitalized than during the Omicron winter wave.

The two doses of vaccines were 24% effective against hospitalizations after their options took over, compared to 61% during the period when the original version of Omicron prevailed. (These figures are for people who received their second dose at least five months ago.)

This decline was likely due in part to the ability of the subvariants to evade people’s immune protection from the vaccine, and in part because unvaccinated people received some protection from earlier infections.

The booster helped a lot, even though the benefits of those extra doses waned over time. Once subvariants became major sources of infection, a third dose of Pfizer or Moderna vaccines raised the vaccine’s effectiveness against hospitalization to 69 percent initially and to 52 percent after four months or more.

The third doses were even more protective during the winter surge of the original version of Omicron.

Second boosters were allowed at the end of March for people 50 and older with healthy immune systems, and these extra boosters appear to help people survive the subvariant surge, the CDC found.

The agency said that at least a week after the fourth dose, the vaccines were 80 percent effective against Covid hospitalizations. This was a significant increase from the 55% efficacy provided by three doses at four months in this age group.

It was unclear how quickly the protection provided by this fourth dose would wear off. The study also did not measure the effectiveness of the vaccines. vs. BA.5, Omicron’s latest sub-variant, which appears to be causing a new surge in cases and hospitalizations. This subvariant has become dominant among new cases in the US and appears to be the most evasive form of Omicron to spread in the country.

As hospitalizations rise, federal health officials urged eligible people to receive booster doses as soon as possible, stating that these shots will not prevent people from getting an extra dose of the updated vaccine against a specific variant in the fall or winter. The agency said the latest results have heightened the need for additional vaccinations.

“Given the recent rise in deaths and hospitalizations associated with option BA.5,” the CDC said on Friday, “everyone should be up to date on recommended Covid-19 vaccinations, including additional booster doses for people who are moderately to severely immunocompromised and adults over 50.”