Pfizer and BioNTech are launching a clinical trial to test an omicron-specific version of their COVID-19 vaccine, the companies announced today. They’re moving forward with studies of a new form of the vaccine in response to concerns that the original shot isn’t preventing infection with the newest variant of coronavirus or mild COVID-19 illness — even though it appears to be holding up well in terms of preventing hospitalization and death, especially for people who received a booster.

The original two-dose series of the gene-based COVID-19 vaccines, made by Pfizer / BioNTech and Moderna, don’t appear to offer much protection against infection with the omicron variant with the virus. A third booster dose restores some of that protection, but it’s unclear how long that protection lasts. Two shots are still somewhat effective against severe illness and death, but the third dose solidifies that protection.

Omicron has a slate of mutations that makes it more difficult for antibodies produced against the original form of the virus, which are the types generated by the vaccines, to block it. The hope is that an omicron-specific form of the vaccine, which should push the body to produce omicron-specific antibodies, would provide longer-lasting protection against infection, mild illness, and serious cases of COVID-19.

The newly launched study will test an omicron-specific shot in 1,420 people. People who have already received two doses of the original vaccine will get an additional one or two doses of the omicron-specific shot. People who have already received three doses of the original will get either an additional dose of the original or one dose of the omicron shot. And a third set of people who have never been vaccinated will get three doses of the omicron-specific shot.

“This study is part of our science-based approach to develop a variant-based vaccine that achieves a similar level of protection against omicron as it did with earlier variants but with longer duration of protection,” said Ugur Sahin, CEO and co-founder of BioNTech, in a statement.