The Pfizer/BioNTech Covid-19 vaccine offers some protection against the new omicron variant, but people might need to get three shots instead of just two to get the same level of protection, based on preliminary results from lab tests conducted by the companies. A fuller picture of the level of protection offered by the messenger RNA vaccine, and whether an omicron-specific version is needed, awaits more data from real-world studies.

Speaking during a Wednesday press conference, BioNTech CEO Ugur Sahin said that that while previous variants have mutations that ranged from 10 to 15 in number, omicron exhibits 34 to 35 mutations. Many of those mutations are on a part of the spike protein called the receptor binding domain—the part that binds to a protein on the surface of a cell in order to enter it, Sahin said.

Some of omicron’s mutations are shared with other variants of the novel coronavirus. That means that the current version of the Covid-19 vaccine still has an effect, but the preliminary data from a small sample size suggest it’s a diminished one. BioNTech has not yet published data that can be peer reviewed. In its Wednesday announcement of the preliminary lab data, BioNTech said it tested blood samples from 19 to 20 people who had received either two shots or three shots of the Covid-19 vaccine. In those who had received two shots, the sample was collected three weeks after the second shot. In subjects that had received a booster—a third shot that followed the initial two-shot vaccination series—the sample was collected after one month.

Each samples was tested simultaneously for neutralizing antibody levels against the spike proteins of both the unmutated virus and the the omicron variant. According to the company, omicron reduced the average levels of neutralizing antibodies by more than 25-fold, indicating that two doses may not provide sufficient protection against the new variant.

In addition to prompting the immune system to produce neutralizing antibodies, Covid-19 vaccines also lead to T cell responses. On this measure, BioNTech said the current version of the vaccine performed better with about 80% of the parts of the spike protein that are recognized by memory T cells unaffected by omicron’s spike protein mutations. Memory T cells are important because these cells remain long after an infection and stand ready to respond quickly if they encounter the  pathogen again.

BioNTech Chief Medical Officer Ozlem Türeci said that the presence of these memory T cells may still protect against severe disease from the omicron variant. BioNTech expects that two doses may offer protection against severe disease, however, a third dose of may be needed to get the protection offered by neutralizing antibodies. Türeci added that these preliminary data “shows us that the first line of defense with two doses of vaccination might be compromised and three doses of vaccination are required to restore protection.”

A variant evaluation study began nine months ago in preparation for a potential change in the vaccine, Sahin said. Clinical trials are ongoing against the beta, delta, and alpha variants. Yet another clinical trial is evaluating a version of the vaccine that protects against both the alpha and delta variants. In tests against the beta variant, the reduction in levels of neutralizing antibodies was fivefold. Sahin said that these results suggest omicron is a much stronger neutralizing antibody escape variant.

“It is still not a complete escape variant; it is a partial escape variant,” he said. “That means the virus can be neutralized by high titers of neutralizing antibodies.”

According to the company, the preliminary data suggest that a booster of the current messenger RNA vaccine increases antibody levels by 25-fold. That additional shot also increases the levels of memory T cells that can offer additional protection from severe disease.

Omicron was first identified in South Africa last month. An omicron-specific version of the Pfizer/BioNTech vaccine has already been designed; if it turns out it is needed, that vaccine can be ready for delivery by March, pending regulatory authorizations, Sahin said. The need for an omicron-specific vaccine will depend on data from real-world studies that show how quickly omicron is spreading as well as how effective the current vaccine performs against the new variant. Getting those data could take another six to eight weeks, Sahin said.

If BioNTech proceeds to roll out an omicron-specific vaccine, Sahin said it should be a three-dose vaccine. That vaccine could also offer protection against more than one variant. Sahin noted that the variant clinical trials underway have already shown that a multivalent vaccine can be produced, but more data about variants are needed to determine whether such a change is necessary.

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