Moderna said people vaccinated in 2020 caught COVID-19 more often than those vaccinated this year.
This suggested immunity from its COVID-19 shot waned over time, and provided evidence for a booster dose, it said.
But the number of people that got severe COVID-19 wasn't significantly different between the groups.
Moderna said new data from its late-stage clinical trial provided proof of waning immunity from its COVID-19 vaccine and a need for a booster dose - but its shot still appears to protect against severe illness.
People who got its vaccine more recently, between December 2020 and March 2021, reported 36% fewer "breakthrough" COVID-19 infections, compared with those vaccinated during its initial trial between July and October 2020, the company said Wednesday, citing an unpublished company analysis.
There were 88 breakthrough cases in the more recently vaccinated group of 14,746 people, and 162 COVID-19 cases in the 11,431 trial participants, eight to 12 months after the second dose, the company said.
Moderna said the findings show evidence of waning immunity from its vaccine, and a "potential benefit" for a booster.
Stephen Hoge, president at Moderna, said on a conference call with investors that "the first six months are great, but you can't count on that being stable out to a year and beyond," as reported by Reuters.
He said that "the estimated impact of waning immunity would be 600,000 additional cases of COVID-19" across the US population of 66 million. "We would expect some of those cases to be severe and some could unfortunately result in death," he said.
But the data also suggested its shot protected against severe COVID-19 long-term. There were only 19 "severe" breakthrough infections overall, and no "significant difference" between the two groups of people, it said. The company didn't define what it meant by "severe" COVID-19, but usually this means illness that kills or requires hospital treatment.
Mild breakthrough infections are expected with any vaccine because no vaccine is 100% effective.
Moderna's announcement comes as the Food and Drug Administration (FDA) is soon to decide whether to give an extra dose of Pfizer's COVID-19 vaccine to fully-vaccinated Americans to shore up immunity against the highly infectious Delta variant, which can evade the immune response.
The FDA is expected to decide on a potential Moderna booster soon after. But some healthcare experts, including Ellie Murray, an epidemiologist at Boston University, have concerns around the quality of data informing the FDA decision: We don't really know how long booster protection lasts, or when best to give a booster. The World Health Organization argues that we should prioritize people worldwide who haven't had a first dose before giving boosters to fully vaccinated people.
The Biden administration hopes that fully vaccinated Americans can get a booster dose at six months after their second dose, from September 20. Immunocompromised people in the US can already get an extra shot.
Moderna vaccine's protection against severe disease
Moderna's shot has performed better than Pfizer's in recent lab studies. Some experts speculated that this could relate to the higher dose of the active ingredient, mRNA, in Moderna's shot, or the longer time intervals between doses - Moderna's vaccine is given four weeks apart, and Pfizer's three weeks.
Moderna's booster shot is half the dose of the original vaccine.
A real-world Qatari study found that Moderna's shot was 84.5% effective against any infection from 14 days after the second dose, including asymptomatic illness. Pfizer's was 53.5% effective, the study found. External factors could influence the results, including that Pfizer's vaccine has typically been given to older people, who tend to produce weaker immune responses.
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