Moderna Says Covid-19 Booster Dose Works Against Omicron in Lab Tests

The new data may reinforce calls by public-health officials for vaccinated people to get booster shots. In the U.S., only about 29.5% of fully vaccinated people have received booster doses, according to the Centers for Disease Control and Prevention.

Moderna, of Cambridge, Mass., said the authorized dosage of its booster shot increased levels of immune-system agents known as neutralizing antibodies against Omicron about 37 times pre-boost levels.

Neutralizing antibodies are among the first soldiers that the immune system deploys to battle invaders like the coronavirus.

“What we showed is when you boost, you get a good brisk increase in antibody levels and they would be correlated with protection,” Moderna Chief Medical Officer Paul Burton said in an interview.

The authorized booster shot is half the dose level used for each of the first two shots of the vaccine.

Together with similar results from lab testing by Pfizer Inc. and partner BioNTech SE, Moderna’s findings suggest Covid-19 vaccines designed to fight the virus strain that was predominant during 2020 may still hold up well against variants such as Omicron that are significantly mutated.

Yet people would have to get a booster shot on top of the primary series of vaccinations to gain the protection.

The Moderna and Pfizer-BioNTech vaccines plus a booster “really do have some substantial protection against Omicron,” said William Schaffner, an infectious-disease specialist at Vanderbilt University Medical Center in Nashville, Tenn. “We would expect that to translate into protection largely against serious disease, to help prevent hospitalization.”

Dr. Schaffner said the lab tests would have to be validated by field studies that show whether vaccines with boosters are actually effective against disease caused by Omicron, and for what duration.

Dr. Schaffner said he thinks “mixing and matching” one company’s booster shot with another for the primary vaccination also is likely to provide protection against severe disease caused by Omicron, though he said more studies would be needed to confirm this.

The vaccines from Pfizer-BioNTech and Moderna, which use messenger RNA, or mRNA, are given in two doses for the primary series, with a third dose being a booster. Johnson & Johnson’s Covid-19 vaccine is given as a single dose, with a second dose as a booster, but U.S. health authorities now recommend the mRNA vaccines should be the preferred choice over J&J’s.

When Omicron emerged, researchers and health authorities expressed concern that the variant could evade vaccines because it has many mutations to the spike protein targeted by the shots.

Adding to their fears was research indicating that Omicron is less susceptible to two doses of Moderna’s and other Covid-19 vaccines.

Lab-test results released last week by a team of researchers from Moderna, the National Institute of Allergy and Infectious Diseases and Duke University showed that two doses of Moderna’s vaccine had significantly reduced neutralization activity against Omicron.

The findings from Pfizer-BioNTech and Moderna are based on lab tests, not the clinical trials in volunteers that are considered more definitive.

Moderna disclosed its booster results, which haven’t been peer reviewed and published in a scientific journal, in a news release. The company plans to share the data with government health officials.

The Omicron variant was identified in late November in southern Africa and has now spread to many countries, including the U.S.

Early studies suggest it spreads faster and reinfects people more easily than other variants and it evades vaccine-induced antibodies to a greater degree. It isn’t known yet whether Omicron causes more or less severe Covid-19.

Moderna also found that using a full dose for the third shot packed a more powerful punch than the authorized booster dose. A third full dose increased neutralizing antibodies against Omicron about 83 times pre-boost levels.

Dr. Burton said government regulators may want to consider recommending the higher-dose boosters to increase protection, at least among people at higher risk of more severe Covid-19.

However, in a separate study, Moderna said people receiving the higher-dose booster had more frequent adverse reactions than those who received the lower dose.

Dr. Burton said the reactions include headache, fever and joint stiffness and should be balanced against the potential benefits of a higher-dose booster.

Moderna also tested other experimental booster shots that target older variants, including Delta, and found they provided a comparable boost to antibody levels against Omicron as its original vaccine booster shot.

Pfizer and BioNTech said a third dose of their Covid-19 vaccine increased neutralizing antibodies against Omicron compared with just two doses.

Given the speed of Omicron’s spread, Moderna said its near-term priority will be to continue making booster shots of its original vaccine available.

It said it would keep working on an Omicron-specific booster shot, in case it is needed. Moderna expects to start clinical trials of an Omicron-specific booster in early 2022.

The new data on Moderna’s booster and Omicron came from lab tests that mix blood samples from vaccinated people with an engineered virus that resembles the Omicron variant. The tests were conducted at Duke University Medical Center labs that were established by the National Institute of Allergy and Infectious Diseases, Moderna said.

Write to Peter Loftus at peter.loftus@wsj.com