Moderna Says Its Vaccine’s Protection Holds After 6 Months

The powerful protection offered by Moderna’s Covid vaccine does not wane in the first six months after the second dose, according to a statement released by the company on Thursday morning in advance of its earnings call.

But during the call, Moderna executives said they anticipated that boosters would be necessary this fall to contend with the Delta variant, which became common in the United States after the results were collected.

“We believe a dose three of a booster will likely be necessary to keep us as safe as possible through the winter season in the Northern Hemisphere,” said Dr. Stephen Hoge, the president of Moderna.

Germany, Israel and France have all decided to administer boosters to potentially vulnerable populations — such as older people or people with compromised immune systems or both — to bolster their immunity in the face of a Delta-driven surge in cases. The Biden administration is considering a similar strategy.

Scientists, though, have not reached a consensus on whether booster shots are needed to boost immunity in fully vaccinated people. On Wednesday, the World Health Organization called for a moratorium on boosters till the end of September. The group urged health leaders to focus instead on vaccinating 10 percent of people in all countries.

Dr. Paul A. Offit, a member of the Food and Drug Administration’s vaccine advisory committee, said that the data presented by Moderna and other vaccine manufacturers so far did not justify rolling out boosters in the next few months. That wouldn’t be needed, he said, unless there was evidence that the vaccines are no longer protecting people against severe disease.

“You want this vaccine to protect against the kind of illness to cause you to seek medical attention, or be hospitalized,” he said. “And until you see any evidence that that isn’t true, then you don’t need a booster dose.”

Moderna’s data came from a new analysis of its clinical trial, which started in late July 2020 and recruited a total of 30,000 volunteers in the United States. In November, the company announced that the vaccine had an impressive efficacy of 94.1 percent. That number didn’t change much after six months, the company reported.

“We are pleased that our Covid-19 vaccine is showing durable efficacy of 93 percent through six months, but recognize that the Delta variant is a significant new threat so we must remain vigilant,” Stéphane Bancel, the chief executive officer of Moderna, said in the statement.

The trial found that the vaccine’s efficacy against severe Covid-19 was 98.2 percent. While three of the volunteers who received a placebo died of Covid-19, none of the vaccinated volunteers did.

“The conclusion we take from these data, is that our efficacy has remained consistently high and durable throughout the period of follow-up,” Dr. Jaqueline Miller, a senior vice president at Moderna, said during in the earnings call.

Last week, Pfizer and BioNTech released a detailed report of their own mRNA vaccine’s durability after six months. The companies estimated that the vaccine’s efficacy started off at 96.2 percent for the first two months after the second dose. It then declined after that, to 83.7 percent by six months.

But experts cautioned that the calculated decline in the Pfizer-BioNTech study could have been a statistical artifact. Chance alone could lead to a different efficacy estimate at different times. “I would not assume waning immunity based on this study alone,” said Maria Deloria Knoll, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

The F.D.A. is expected to give full approval to the Pfizer-BioNTech vaccine next month. Moderna filed for final approval of its vaccine on June 1, and expects to complete its submission in August. Pfizer is also expected to ask the F.D.A. to authorize a booster dose this month.

To estimate the efficacy of their vaccine, Moderna researchers looked at data from the trial up until late March. The Delta variant did not become common in the United States until weeks later. As a result, they cannot determine how well the vaccine protects people against Delta based on the clinical trial.

In June, Moderna released details on an experiment in which its researchers tested antibodies from people who received their vaccine against the Delta variant. They found that the antibodies were moderately less effective at blocking the variant from infecting cells.

In the earnings call on Thursday, the company presented details from additional studies. They found that the strength of the antibodies against variants waned substantially by six months after the second dose.

Moderna has been developing a range of boosters and testing them in clinical trials. On Thursday the company reported that a booster containing half a dose of the original formulation strengthened the antibodies against the Delta variant substantially above the levels seen shortly after volunteers received their two original doses.

The waning antibodies combined with the recent surge of infections would call for a booster, Dr. Hoge said in the earnings call. “We believe a booster dose is likely to be necessary this fall, particularly in the face of the Delta variant,” he said.

Rebecca Kahn, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health, disagreed.

“These Phase 3 results underscore that the vaccines are highly effective,” she said. “While it’s important to continue evaluating their effectiveness against new variants, the priority needs to be increasing global access to first and second doses.”

In an earnings call last week, Pfizer also said that its booster raised antibodies above their original level. The booster studies from both companies have yet to be published in a scientific journal.