The four studies—conducted by South African, Swedish, German, and American researchers—indicate two shots of the vaccine aren’t as effective in stopping the Omicron variant. The scientists recommend booster shots to increase protection.

While the studies don’t offer a direct measurement of vaccine effectiveness, scientists use the number of remaining antibodies, which inhibit viral entry into cells, to get a rough idea of how steep the reduction in vaccine effectiveness will be, says Ben Murrell, a researcher who worked on one of the antibody studies at Stockholm’s Karolinska Institute.

The results of the studies varied, with two finding a 40-fold reduction in the levels of neutralizing antibodies and the other two studies finding up to a 25-fold reduction, but all concluded that people with two vaccine jabs have weaker immunity against the Omicron variant than against the other variants in circulation.

“The direction of the effect was consistent between the three [now four] studies. All studies show this is worse than Delta,” Murrell told Fortune. “The magnitude of your antibody potency matters, and that’s what you can boost with existing vaccines.”

The new variant has been found in at least 45 nations and has proved itself to be highly contagious but mildly symptomatic.

A 40-fold reduction

The first look into vaccine effectiveness against the new Omicron variant came from a 12-participant South African study conducted by the Africa Health Research Institute in Durban. Researchers found a 40-fold reduction in levels of neutralizing antibodies produced by people with two shots of the Pfizer-BioNTech shot compared with the first strain of COVID-19 detected in Wuhan, China. The researchers suggested a third dose would help fight the highly mutated strain.

The loss of immune protection is “robust, but not complete,” Alex Sigal, head of research at the laboratory, said in an online presentation.

A booster shot of Pfizer and the two shots of Pfizer with a previous infection seems to give similar amounts of immunity.

The other study that found a 40-fold decline in antibodies was conduced by German virologist Sandra Ciesek, who published the study on Twitter. Ciesek wrote on Twitter that the development of a new Omicron-targeting booster shot makes sense, and her data “cannot say anything about whether you are still protected against a severe course.”

A 25-fold reduction

A separate study done at Stockholm’s Karolinska Institute found antibodies declined by up to 25-fold against a pseudoviral strain of Omicron. There was great variation across the study, however, with some people experiencing very little change in antibody levels. Testing blood samples from a cohort of 34 people, the researchers noted that previous exposure history was key to vaccine effectiveness.

The most recent study to emerge was conducted by drugmakers Pfizer and BioNTech, who also found that individuals with only two doses of their vaccine had around a 25-fold reduction in antibody levels against the Omicron variant compared to an earlier version of the virus. The companies noted that the finding indicates that two doses alone “may not be sufficient to protect against infection” by the new variant.

Further backing the argument for boosters, the blood samples from people one month out from their booster shot showed neutralizing antibodies against the Omicron variant were comparable to levels of antibodies against a previous strain of the virus after two doses, the companies said in a statement.

Vaccines tweaks

Vaccine makers are already preparing for waning vaccine immunity when faced with the Omicron variant.

BioNTech and Pfizer said that in the case of a dominant strain of the virus emerging that evades the vaccine or reinfection immunity, otherwise known as an “escape variant,” the vaccine manufacturers could adapt their mRNA vaccine within six weeks and start shipping doses of boosters within 100 days.

While no news has emerged on Moderna vaccine’s immunity against the new strain, the drugmaker is similarly developing an Omicron-targeting booster mRNA vaccine that could be ready for clinical testing within 60 to 90 days. Moderna is also looking at two “multivalent” boosters, which would target common mutations seen across multiple variants.

“I think that there’s a real risk that we’re going to see a decrease in the effectiveness of the vaccines,” Moderna president Stephen Hoge said in an interview with ABC. “What I don’t know is how substantial that is.”

Johnson & Johnson is pursuing its own Omicron-targeted one-shot vaccine that will be developed as needed, the company said in a statement. “We will not be complacent,” Dr. Mathai Mammen, J&J’s global head of research and development, said in a statement.