Glass vials of Moderna's Covid-19 vaccine are seen in April at a vaccine clinic in Pennsylvania.
CNN  — 

Moderna’s Covid-19 vaccine showed 93% efficacy against symptomatic disease through six months, and the company expects to complete its application for full approval from the US Food and Drug Administration this month, the company said Thursday.

The efficacy data came from a final analysis of the vaccine’s Phase 3 study, which enrolled thousands of participants who received both doses last year, before it was made available to the wider public.

“In final analysis” of the study, “the Moderna Covid-19 vaccine showed 93% efficacy, with the efficacy remaining durable through six months after administration of the second dose,” Moderna said in a news release.

The data reflecting 93% efficacy was from trial participants who had been fully vaccinated by November 25 and evaluated for the following six months, before the highly contagious Delta variant became dominant in the United States and spurred the current surge in cases.

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

Last month, the makers of the other mRNA vaccine authorized in the United States, Pfizer/BioNTech, released efficacy data showing an overall six-month efficacy of 91% – with possible waning toward the end of that time. Like the Moderna data, the Pfizer follow-up data was collected prior to the Delta-related surge.

Pfizer’s pre-print paper showed its vaccine’s efficacy – in a trial involving 44,000 volunteers around the world – peaked at more than 96% from a week to around two months after a second dose, and then appeared to gradually decline to 83.7% four to six months later, according to the paper, which had not yet been peer-reviewed or published in a journal.

Moderna says boosters may be needed before winter

Moderna believes the Delta variant will lead to more breakthrough infections, and that boosters may be needed before winter, according to slides published ahead of the company’s earnings call on Thursday.

“We believe that increased force of infection resulting from Delta, non-pharmaceutical intervention (NPI) fatigue, and seasonal effects (moving indoors) will lead to an increase of breakthrough infections in vaccinated individuals,” one of the slides says.

Moderna also said that while it sees durable efficacy of the vaccine, it expects neutralizing titers will continue to wane and eventually impact the efficacy of the vaccine.

“Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season,” a slide says.

Moderna is waiting for dosage data before filing for an emergency use authorization (EUA) for a booster shot, Moderna president Dr. Stephen Hoge said in Thursday’s investor call.

Moderna’s first two doses are 100 micrograms each. The company says it is exploring whether a booster – a third shot of the same vaccine – should be 50 micrograms or 100 micrograms.

“Our clinical data right now, we think, supports a 50-microgram … booster,” Hoge said. “But, we’re going to wait for 100-microgram data in the coming weeks to confirm the dose selection of 50 micrograms as the booster before filing.”

In Thursday’s call, Hoge presented early data, yet to be peer reviewed, showing the vaccine’s neutralizing antibodies waning six months after the second dose – including against the Delta variant.

Hoge said the yet-to-be-peer-reviewed data showed that 14 days after a 50-microgram booster was administered, antibody titers increased significantly.

Hoge called this “very encouraging,” adding that the company believes this confirms its selection of the booster as likely to be protective against circulating variants of concern, particularly Delta.

Experts have said it is unclear how these antibody levels correlate with real-world immunity, and to what extent other parts of the immune system – such as T cells – could factor into protection.

Pfizer said in an investor call last week that it intends to submit a booster for EUA consideration as early as August.

The FDA and US Centers for Disease Control and Prevention said at the time Americans don’t need third doses quite yet, and that it was not up to companies alone to decide when an additional dose might be needed.

Dr. Francis Collins, director of the National Institutes of Health, told CNN on Tuesday, “At the present time … the data in the United States does not indicate that (a booster is) necessary.”

In July, the maker of the third Covid-19 vaccine authorized in the United States – Johnson & Johnson – said its one-dose product provides immunity that lasts at least eight months, and it appears to provide protection against the Delta variant.

On Tuesday, the San Francisco Department of Public Health announced it would provide people who received the one-dose Johnson & Johnson vaccine with a supplemental dose of an mRNA vaccine, either Pfizer or Moderna. It would give these supplemental doses to those who have consulted with their doctor beforehand.

Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said Tuesday the “FDA does not recommend taking things into your own hands” regarding Covid-19 vaccine booster shots.

“You can see all from looking at the news that there are people and the jurisdictions that are actually taking things into their own hands. … FDA does not recommend taking things into your own hands,” said Marks in a discussion hosted by the Covid-19 Vaccine Education and Equity Project briefing.

The San Francisco health department maintains that it aligns with the CDC and FDA. “We are not recommending; we are accommodating requests,” Dr. Naveena Bobba, deputy director of health for SFDPH, said at a media briefing Tuesday. “We have gotten a few requests based on patients talking to their physicians and that’s why we are allowing for the accommodation.”

CNN’s Jacqueline Howard contributed to this report.