CNN  — 

The updated Covid-19 booster shots appear to work about as well against the BA.4 and BA.5 Omicron subvariants as the original boosters they replaced, according to two new studies from research teams at Harvard and Columbia universities.

The research suggests that our bodies have been well-trained to fight the original virus, which emerged from Wuhan, China, and that boosters mostly reinforce that response. Getting boosted this fall is still an important way to renew protection, even among people who were previously infected or vaccinated.

But the hope was that by tweaking the vaccine recipe to include currently circulating strains of the Omicron variant, it would help broaden immunity against those variants and perhaps offer better and longer-lasting protection.

When the researchers compared the immune responses of people who got a booster dose of the original shot to people who got the updated bivalent boosters, they looked about the same.

“We see essentially no difference” between the old boosters and the new about a month after the shot, said Dr. David Ho, professor of microbiology and immunology at Columbia, whose team authored one of the studies.

Immunologists say a vaccine against two strains may not be better than a single strain shot because of a phenomenon called immune imprinting. Scientists say imprinting may complicate efforts to stay ahead of new variants as the coronavirus continues to evolve, and it adds urgency to the development of new vaccine technologies to fight the virus.

When the US Food and Drug Administration issued emergency use authorizations for new bivalent Covid-19 vaccines from Pfizer and Moderna at the end of August, it did so on the basis of studies in mice and previous human trials with a different two-strain booster formulation. Little was known about the how protective the shots might be in people; full data from clinical trials testing the BA.4 and BA.5 bivalent vaccines in humans hasn’t yet been made public.

But modeling data suggested that getting the boosters out in September could save tens of thousands of lives if the country had another winter surge, so the FDA authorized the shots, ahead of results from clinical trials, in order to get them to the public more quickly.

The updated shots contain instructions that show cells how to make spike proteins from the original virus that caused Covid-19, as well as spikes from the BA.4 and BA.5 subvariants. These spikes get assembled by our cells and displayed to our immune system so it can make antibodies to fight the real thing during an active infection.

The original strain of the virus, sometimes called the ancestral or wild-type strain, is no longer circulating, however. When we boost, we are mostly boosting antibodies against a virus that’s long gone.

As the virus has evolved, the vaccines have not kept pace. Each new variant has become more and more resistant to the antibodies we make against it, increasing the risk of breakthrough infections, hospitalizations and deaths.

Right now, protection against infection begins to wane just a few months af