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Mayo Clinic: Don't jump to conclusions about vaccine effectiveness study

The study found that vaccines are effective at preventing severe disease, but perhaps not as strong at infection prevention.

The Mayo Clinic has cautioned people not to jump to conclusions after results from its preprint study – which has not yet been peer reviewed – into the effectiveness of mRNA COVID-19 vaccines were released.

The study of more than 25,000 people between January and July found that the vaccines from both Pfizer/BioNTech and Moderna remain highly effective at preventing hospitalization from the delta variant of COVID-19, but suggests a declining effectiveness after 6 months at preventing infection, particularly the Pfizer shot.

The study has not been peer reviewed and Mayo Clinic cautions drawing conclusions about vaccine efficacy after the report was picked up by some media outlets.

“Mayo Clinic is aware of media reports on the scientific preprint paper comparing vaccines’ clinical effectiveness against the delta variant and breakthrough infections," a statement from Mayo Clinic says. "We caution against drawing conclusions about vaccine effectiveness from a preprint study, which is intended only to be helpful to the scientific community and has not yet undergone the rigor of peer review.”

According to the study, the prevalence of the delta variant at Mayo Clinic Health Systems in Minnesota, Wisconsin, Iowa, Florida and Arizona in July showed that infection prevention dropped in those who had received the Pfizer or Moderna vaccines more than six months ago.

The effectiveness of Moderna vaccine at preventing infection dipped to a (still very effective) 76% in July, while Pfizer infection prevention dropped to 42%. In Florida, the preprint study found that the risk of infection in July for fully vaccinated Moderna recipients was about 60% lower than for people who were vaccinated with Pfizer.

But encouragingly, both vaccines are still extremely effective at preventing serious illness from COVID-19 in excess of six months.

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In any case, the Mayo study found that "both mRNA COVID-19 vaccines strongly protect against infection and severe disease," but that the preliminary findings suggest  more evaluation is needed as to "dosing regiments and vaccine composition," which comes as the FDA is considering approving boosters of Pfizer and Moderna for vulnerable populations.

In Minnesota, 57.1% of those vaccinated received Pfizer shots, with 38.4% getting Moderna. A smaller number of 4.5% got the one-dose Johnson & Johnson shot, which isn't a mRNA vaccine.

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Florida's situation is night and day compared to Minnesota. According to the Miami Herald, Florida reset its record for current COVID-19 hospitalizations for an 11th consecutive day Wednesday, with 15,449 people hospitalized. More than 3,100 patients are in intensive care, accounting for 47.5% of all ICU patients in Florida. 

Through Wednesday, the Minnesota Department of Health reported 377 people with COVID-19 admitted to hospitals, including 103 in intensive care. Hospitalizations have been steadily rising with the delta variant becoming the dominant strain since July. 

Comparing Minnesota to Florida is difficult, however, because Minnesota has a higher vaccination rate, fewer people – 21.4 million people in Florida, 5.6 million in Minnesota – and is less densely populated. 

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