Story at a glance

  • Researchers from data analytics company nference compared the effectiveness of both the Moderna and Pfizer vaccines from January through July in Mayo Clinic patients.
  • Researchers found the efficacy of Moderna’s vaccine in the study fell from 86 percent in early 2021 to 76 percent in July.
  • During the same time period, however, Pfizer’s vaccine effectiveness fell from 76 percent to 42 percent.

While all COVID-19 vaccines being administered in the U.S. have shown to dramatically reduce the risk of infection, severe disease and death, one mRNA vaccine may be more effective than the other when it comes to the highly contagious delta variant, according to a study that has yet to be peer-reviewed.

Researchers from data analytics company nference compared the effectiveness of both the Moderna and Pfizer vaccines from January through July. The study posted Sunday included more than 50,000 patients in the Mayo Clinic Health System, according to Reuters. 

The observational study found both vaccines were highly effective against COVID-19 infection and hospitalization, but their efficacy began to wane in July when delta became the dominant strain of the virus in the U.S. 

Researchers found the efficacy of Moderna’s vaccine in the study fell from 86 percent in early 2021 to 76 percent in July. During the same time period, however, Pfizer’s vaccine effectiveness fell from 76 percent to 42 percent. 

Authors of the research emphasized that both vaccines are highly effective at preventing hospitalization due to COVID-19, but a Moderna booster shot may be needed for those who received any mRNA vaccine earlier this year, according to Reuters. Pfizer has said a third dose of vaccine may be needed within 6 to 12 months after full vaccination. 


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A separate study that hasn’t been peer-reviewed also found elderly nursing home residents in Ontario produced stronger immune responses with the Moderna vaccine than the Pfizer jab, particularly with regard to variants of concern, Reuters reports. 

It’s worth emphasizing that other research has shown Pfizer’s vaccine works incredibly well against delta. 

Research published in the New England Journal of Medicine just last month found that two doses of the Pfizer vaccine provide 88 percent protection against delta. The findings reinforced earlier data from England's Public Health department in May that reported the same level of protection. 

In clinical trials, Pfizer’s vaccine showed to be 95 percent effective in preventing symptomatic COVID-19 and Moderna’s was 94 percent effective. 


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Published on Aug 10, 2021