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COVID & Vaccine Efficacy

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8/16/21—According to a new Mayo Clinic study, Pfizer’s BioNTech vaccine, the most commonly used COVID-19 vaccination in the United States, is only 42% effective1 against the Delta variant. Data suggests the Moderna vaccination gives more protection against this particular strain, noting 76% accuracy when compared with the original strain. This study of breakthrough coronavirus cases utilized PCR testing data from Mayo Clinic Health System’s sites in five states, Minnesota, Wisconsin, Arizona, Florida and Iowa.

Delta has quickly become the dominant strain in the U.S., with the Centers for Disease Control and Prevention (CDC) indicating 1 that vaccinated and unvaccinated individuals release similar viral loads when infected with the variant. While vaccine efficacy has shown a decrease in ability to combat Delta, study data 2 has shown Pfizer is highly effectively against hospitalizations, at 85%, and Moderna at 91.6%.  Of the over 60 thousand individuals involved in the study, at least 40% were over the age of 65, with no COVID-19 breakthrough deaths within any age group occurring during the course of the study.

While the vaccines initially rated as largely effective earlier in the year, those numbers started dropping as the Delta variant surged from .7 to over 70% in July. While the data from this study was not sequenced 1 to see exactly how many of the breakthrough cases were Delta and how many were other strains, the lead author of the study, Venky Soundararajan, believes both reduced efficacy against Delta and loss of vaccine potency over time are at play here. “The Moderna vaccine is likely – very likely – more effective than the Pfizer vaccine in areas where Delta is the dominant strain, and the Pfizer vaccine appears to have a lower durability of effectiveness,” Soundararajan said. He also noted that additional study was needed, as this study is yet to be peer reviewed.

Hoping to curb spread and reduce the production of additional variants, the FDA is planning 3 to approve a third vaccine dose for immunocompromised individuals, with the possibility of booster shots for all Americans looking more likely than ever. Pfizer has announced 1 that as soon as it receives regulatory approval, they intend to develop a vaccine specifically designed to combat the Delta variant within 100 days.

As many return to schools and offices this fall, it is clear that while vaccination is essential, it remains only one facet of the layered approach necessary to mitigate spread most effectively. At the end of July, the CDC updated their guidance for fully vaccinated individuals in light of the Delta variant. This guidance 4 not only recommends masking in areas of substantial spread, but also calls for the testing, masking, and isolation of any close contacts of confirmed cases. In order to identify these individuals and end the chain of transmission, contact tracing is a key. With spread as transmissible as we’ve seen with Delta, the digital solution from AlertTrace can quickly and easily help businesses and organizations stay proactive about prevention. Contact us today to learn more.

1 Shaheen, Mansur. “Pfizer’s Covid Vaccine Is Only 42% Effective against Indian ‘Delta’ Variant While Moderna’s Jab Is 76% Effective, Mayo Clinic Study Suggests.”, 11 Aug. 2021,

2 Collman, Ashley. “Pfizer’s Vaccine May Be Less Effective against the Delta Variant, Study Suggests, Even Though Vaccines Help Prevent Serious Illness or Death.” MSN, 12 Aug. 2021,

3 Edwards, Erika, and Heidi Przybyla. “FDA Poised to OK 3rd Vaccine Dose for Immunocompromised People.” NBC News, 11 Aug. 2021,

4 “Interim Public Health Recommendations for Fully Vaccinated People.” Centers for Disease Control and Prevention, 28 July 2021,