In-Depth: Is the Moderna vaccine better than Pfizer's?

New UCSD study offers clues
Virus Outbreak Vaccine
Posted at 6:09 PM, Sep 02, 2021
and last updated 2021-09-02 21:43:09-04

SAN DIEGO (KGTV) -- Several new studies suggest the Moderna COVID-19 vaccine maintains a higher level of protection than the one from Pfizer several months after immunization, but experts say the differences may have more to do with the interval between doses rather than the shots themselves.

In a new study published in the New England Journal of Medicine, UC San Diego Health showed the effectiveness of mRNA vaccines among its 19,000 healthcare workers dropped to 65 percent in July. It had been 93 percent or higher from March through June.

The authors attributed the rapid decline to the emergence of the delta variant coupled with the end of California’s mask mandates and business restrictions on June 15.

The mRNA vaccines continued to provide strong protection against severe disease. The only employee who had to be treated in the hospital was unvaccinated.

The published study did not differentiate efficacies by manufacturer, but UCSD shared updated data with ABC 10News showing the decline was more significant in Pfizer recipients than those who got Moderna.

Both shots were about 95 percent effective in June. In the first 26 days of August, Moderna declined to 65 percent. Pfizer dropped to 49 percent.

UCSD’s data mirrors other findings across the country. The Mayo Clinic released a study showing Moderna’s vaccine was 76 percent effective in July. Pfizer’s was just 42 percent.

Across several states, the authors found people with Pfizer were about twice as likely to have a breakthrough infection during the study period than people with Moderna. Both vaccines provided strong protection against hospitalization. The paper has not yet undergone peer review.

This week, a study in Belgium showed the Moderna vaccine produces 2.5 times as many virus-fighting antibodies as the Pfizer vaccine.

So is Moderna officially better?

“My suspicion personally is that we’re going to find it’s more about the dose gap than the vaccine itself,” said Dr. Christopher Longhurst, one of the authors of the UCSD study.

Pfizer and Moderna use two doses of mRNA technology, a blueprint of the virus’s spike protein. Moderna decided to separate the doses by four weeks instead of three.

La Jolla Institute for Immunology professor Dr. Shane Crotty said many vaccines work better with a longer interval.

“After that first immunization, your immune system is essentially still learning about the target for quite a while. And it’s not done learning at three weeks. And it’s not done learning at five weeks,” he said.

Crotty said the white blood cells responsible for making antibodies evolve, and they do a better job with more time.

RELATED: In-Depth: Delaying the second COVID-19 vaccine dose has benefits and drawbacks

The interval isn’t the only difference between the vaccines. Moderna’s vaccine has a higher concentration of mRNA, 100 micrograms compared to 30. But several studies have revealed the critical importance of timing.

Researchers in the U.K. showed a longer interval between doses of the AstraZeneca vaccine increased its efficacy. In an early clinical trial, the vaccine’s efficacy in adults who got their second dose after 12 weeks was 81.3 percent. When the second dose was administered in less than six weeks, it was 55.1 percent.

Another U.K. study showed Pfizer’s vaccine produces 3.5 times as many antibodies when the second dose is adminstered at 12 weeks.

The findings were hailed as justification for the U.K.’s policy to delay second doses for its citizens. Scientists are still studying the effects.

“I wouldn’t be surprised if the U.K. finds with a 12-week gap between first and second doses both [the Pfizer and Moderna] vaccines are equally effective,” Longhurst said. “The U.K. may not even need booster doses because they get a better immune response after three months.”

Here in U.S., Dr. Longhurst wants to see booster doses for healthcare workers right away.

Even though the vaccines continue to prevent severe disease well, he said their employees can’t afford mild infections that force them into quarantine for ten days or more.

It keeps hospital workers away from the hospital.