• Moderna CEO Stephane Bancel said the efficacy of boosters against Covid-19 will likely decline over time, and some people may need a fourth shot by the fall.
• "I would expect that it's not going to hold great," Bancel said, during a Goldman Sachs interview, referring to the strength of the single booster shots.
• A random mutation could change the course of the pandemic again, Bancel warned.

Moderna CEO Stephane Bancel on Thursday said the efficacy of boosters against Covid-19 will likely decline over time, and people may need a fourth shot in the fall to increase their protection.

Bancel said people who received their boosters last fall will likely have enough protection to get them through the winter, when new infections surge as people gather indoors to escape the cold.

However, Bancel said the efficacy of boosters will probably decline over the course of several months, similar to what happened with the first two doses. The Moderna chief was interviewed by Goldman Sachs during the investment bank's health-care CEO conference.

"I will be surprised when we get that data in the coming weeks that it's holding nicely over time — I would expect that it's not going to hold great," Bancel said, referring to the strength of the booster shots.

An unprecedented surge of infections from the highly contagious omicron variant is currently spreading worldwide. In the U.S., the seven-day average is now more than 574,000 new cases daily, according to a CNBC analysis of data from Johns Hopkins University.

The Moderna CEO said governments, including the U.K. and South Korea, are already ordering the doses in preparation. "I still believe we're going to need boosters in the fall of '22 and forward," Bancel said, adding that people who are older or have underlying health conditions might need annual boosters for years to come.

"We have been saying that we believe first this virus is not going away," Bancel said. "We're going to have to live with it."

Moderna published preliminary data last month that showed its currently authorized 50 microgram booster shot increased the antibodies that block infection from omicron 37-fold. A 100 microgram booster increased those antibodies 83-fold.

Booster shots are playing an increasingly important role in public health strategies to control the virus, with protection from the original two shots having taken a significant blow from omicron.

Data from the United Kingdom found that Moderna and Pfizer's two-dose vaccines are only about 10% effective at preventing symptomatic infection from omicron 20 weeks after the second dose.

The same study, published by the U.K. Health Security Agency, found that booster doses are up to 75% effective at preventing symptomatic infection two weeks after receiving the shot.

However, the efficacy of booster shots starts to decline after about four weeks, according to the study. Boosters were 55% to 70% effective at preventing infection at weeks five to nine, and 40% to 50% effective 10 weeks after receiving the shot.

Pfizer CEO Albert Bourla told CNBC last month that people will likely need a fourth dose, and the shot may be needed sooner than expected due to omicron's virulence.

Bancel, during the Goldman Sachs interview, said omicron could accelerate the transition from the acute crisis caused by the virus to an endemic phase where enough people have immune protection so that Covid isn't as disruptive to public life.

However, he also cautioned against predictions, noting that omicron, with its dozens of mutations, took most of the scientific community by surprise. The data so far indicates that omicron is more transmissible but less severe than past strains.

However, a random mutation could change the course of the pandemic again, Bancel said.

"What is totally impossible to predict, is there a new mutation coming in a day, a week, three months that is worse in terms of severity of disease," he said. "That's a piece that we'll have to just be cautious about."