Coronavirus: Vaccines
Moderna vaccine seems to have slight edge over Pfizer's: Studies
Both are still highly effective at preventing severe illness and hospitalisation, say experts
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NEW YORK • It was a constant refrain from federal health officials after the coronavirus vaccines were authorised: These shots are all equally effective. That has turned out not to be true.
Roughly 221 million doses of the Pfizer-BioNTech vaccine have been dispensed in the United States, compared with about 150 million doses of Moderna's vaccine. In a half-dozen studies published over the past weeks, Moderna's vaccine appeared to be more protective than Pfizer's vaccine months after immunisation.
The latest study, published on Wednesday in The New England Journal of Medicine, evaluated the real-world effectiveness of the vaccines at preventing symptomatic illness in about 5,000 healthcare workers in 25 states. The study found the Pfizer shot had 88.8 per cent effectiveness, compared with Moderna's 96.3 per cent.
Research published last week by the Centres for Disease Control and Prevention found the efficacy of the Pfizer-BioNTech vaccine against hospitalisation fell from 91 per cent to 77 per cent, four months following the second shot.
Moderna's vaccine was 93 per cent effective at reducing the short-term risk of Covid-19 hospitalisation and remained 92 per cent effective after 120 days.
If the efficacy gap continues to widen, it may have implications for the debate on booster shots. Federal agencies are evaluating the need for a third shot of the Pfizer-BioNTech vaccine for high-risk groups, including older adults.
Scientists who were sceptical of the reported differences between the Moderna and Pfizer vaccines have become convinced that the disparity is small but real.
"Our baseline assumption is that the mRNA vaccines are functioning similarly, but then you start to see a separation," said Dr Natalie Dean, a biostatistician at Emory University. "It's not a huge difference, but at least it's consistent."
But the discrepancy is small and the real-world consequences uncertain because both vaccines are still highly effective at preventing severe illness and hospitalisation, she and others pointed out.
"Yes, likely a real difference, probably reflecting what's in the two vials," said Dr John Moore, a virus expert at Weill Cornell Medicine. "But truly, how much does this difference matter in the real world? It's not appropriate for people who took Pfizer to freak out that they got an inferior vaccine."
The two shots rely on the same mRNA platform. In initial clinical trials, they had remarkably similar efficacies against symptomatic infection: 95 per cent for Pfizer and 94 per cent for Moderna.
The subtleties emerged over time. The vaccines have never been directly compared in a carefully designed study, so the data indicating that effects vary are based mostly on observations.
Results from those studies can be skewed by any number of factors, including the location, age of population vaccinated, when they were immunised and the timing between the doses, Dr Dean said.
But the observational studies have since delivered results from a number of locations - Qatar, the Mayo Clinic in Minnesota, several other states in the US - and in healthcare workers, hospitalised veterans or the general population.
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How effective the vaccines are in two areas
PREVENTING SYMPTOMATIC ILLNESS
(The New England Journal of Medicine)
Pfizer-BioNTech
88.8%
Moderna
96.3%
PREVENTING HOSPITALISATION FOUR MONTHS AFTER SECOND DOSE
(US Centres for Disease Control and Prevention)
Pfizer-BioNTech
Fell from 91% to 77%
Moderna
Fell from 93% to 92%
Moderna's efficacy against severe illness in those studies ranged from 92 per cent to 100 per cent. Pfizer-BioNTech's numbers trailed by 10 to 15 percentage points.
The two vaccines have diverged more sharply in their efficacy against infection. Protection from both waned over time, particularly after the arrival of the Delta variant, but the Pfizer vaccine's values fell lower. In two of the recent studies, the Moderna vaccine did better at preventing illness by more than 30 percentage points.
Still, experts said the corpus of evidence pointed to a disparity worth exploring, at least in people who respond weakly to vaccines, including older adults and immunocompromised people.
"At the end of the day, I do think there are subtle but real differences between Moderna and Pfizer," said Dr Jeffrey Wilson, an immunologist and physician at the University of Virginia who co-authored one such study published this month.
"In high-risk populations, it might be relevant. It'd be good if people took a close look. Pfizer is a big hammer, but Moderna is a sledgehammer."
In the middle of a pandemic last year, the companies had to guess at the optimal dose. Pfizer went with 30 micrograms, and Moderna with 100. Moderna's vaccine relies on a lipid nanoparticle, which can deliver a larger dose.
Moderna's team recently showed that a half dose of the vaccine still sent antibody levels soaring. Based on those data, the company asked the Food and Drug Administration this month to authorise 50 micrograms, the half dose, as a booster shot.
NYTIMES
