Pfizer COVID-19 vaccine just 42% effective against infection amid delta spread, preprint suggests

COVID-19 vaccine findings were posted ahead of peer review

Early findings posted ahead of rigorous peer review suggested the Pfizer COVID-19 vaccine declined to 42% effectiveness against infection amid sweeping spread of the delta variant, with the Moderna vaccine declining to 76%.

Researchers at the Mayo Clinic and Cambridge-based biotech company nference posted the retrospective study in medrxiv this week, drawing from tens of thousands PCR tests conducted at the Mayo Clinic and affiliated hospitals across nearly half a dozen states.


The study period analyzed samples from January to July, when either the alpha or delta variant was a highly prevalent strain; "the Delta variant prevalence in Minnesota increased from 0.7% in May to over 70% in July whereas the Alpha variant prevalence decreased from 85% to 13% over the same time period," the preprint reads.

While results indicated the Pfizer and Moderna vaccines were highly effective against COVID-19 infection (76%, 86%, respectively) and hospitalization (85%, 91.6%) the effectiveness against infection dropped by July to 42% and 76%, respectively.

Further findings indicated that people vaccinated with  Moderna’s shot were half as likely to develop a breakthrough infection, compared to recipients of the Pfizer vaccine. What’s more, researchers found that Florida residents fully vaccinated with Moderna’s shot faced a 60% lower risk of infection when compared to Pfizer recipients. Also, across all states, the Moderna vaccine cut COVID-related hospitalizations by about half the rate compared to the Pfizer vaccine. Researchers noted "no significant difference" between the groups relating to ICU admission for COVID-19.

In a statement sent to Fox News, Pfizer reiterated confidence in the protection and safety of its two-dose COVID-19 vaccine, and noted an ongoing "robust booster research program to ensure" the vaccine offers the greatest level of protection as possible. Pfizer intends to share the compiled data with regulators around the world in the coming weeks.

"The vaccine continues to be highly efficacious in preventing COVID-19, including variants and to date, no variant, including Delta, appears to have escaped the protection of the vaccine," the statement reads. Fox News has also requested comment from Moderna.


Dr. Gregory Poland, professor of medicine, infectious diseases and molecular pharmacology at the Mayo Clinic, whose name doesn't appear on the study, told Fox News: "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."

Study authors suggested several factors behind the differences observed among the Pfizer and Moderna vaccines, including the dosing, meaning "each mRNA-1273 [Moderna] dose provides three times more mRNA copies of the Spike protein than BNT162b2 [Pfizer], which could result in more effective priming of the immune response."

The study had its limitations; the sample size wasn’t representative of the U.S. population, and authors said other factors could've impacted the findings regarding reduced vaccine effectiveness (like "waning immunity over time, the dynamic landscape of SARS-CoV-2 variants, or other factors that were not considered here.")


"Our observational study suggests that while both mRNA COVID-19 vaccines strongly protect against infection and severe disease, there are differences in their real-world effectiveness relative to each other and relative to prior months of the pandemic," study authors concluded. 

Researchers called for larger studies with more diverse participants to guide public health decisions, including timing of booster doses.