Less Breakthrough Infections with Moderna Than Pfizer

Moderna Vaccine
Source: Reuters

A study of more than 50,000 patients found that the Moderna vaccine provides better protection against breakthrough infections than Pfizer.

As the transmissible Delta variant spreads across the world, countries are facing a new, deadlier wave of infections. Although more than 90% of the new cases have occurred in unvaccinated individuals, a small percentage of cases have also shown up in vaccinated people. This occurrence of infections in fully vaccinated individuals is what we call breakthrough infections. Since no vaccine is 100% effective, breakthrough infections are an expected occurrence. However, it has led to a bit of scepticism among people regarding vaccines. Especially, in light of the dominant delta variant.  

Recently, a study found that mRNA-based vaccines such as Pfizer and Moderna, provide greater protection against delta variant infections than other available COVID vaccines. However, researchers have reported a reduction in vaccine efficacy against the new variant. Thus, raising concerns about the need for a potential booster shot in the future.

In a bid to further assess Moderna and Pfizer’s effectiveness, researchers at Mayo Clinic conducted a study comprising of more than 50,000 participants. They aimed to compare the rates of infection and complications between individuals vaccinated with Pfizer and those that received Moderna. The multi-state Mayo Clinic Health System assessed vaccine effectiveness from January to July 2021. During this time, either the Alpha or Delta variants was dominant in the United States.

Pfizer vs Moderna

Although both vaccines remained highly effective against infection and hospitalization, the degree of protection varied. Moreover, the effectiveness for both vaccines declined in July; a time when delta variant prevalence increased across states.   

According to results, the Moderna vaccine had an overall effectiveness of 86% against infection, and Pfizer had a 76% effectiveness. However, this effectiveness declined as the delta variant spread across the states. In Minnesota, the researchers noted a drop in Moderna’s effectiveness to 76%, while the Pfizer vaccine saw a sharp decline to 42%. Furthermore, in Florida, Moderna recipients had a 60% lower risk of infection in July as compared to Pfizer recipients.

Comparing rates of infection between matched individuals fully vaccinated with mRNA-1273 [Moderna] versus BNT162b2 [Pfizer] across Mayo Clinic Health System sites in multiple states (Minnesota, Wisconsin, Arizona, Florida, and Iowa), mRNA-1273 conferred a two-fold risk reduction against breakthrough infection compared to BNT162b2.

study authors

It is unclear whether the results are evidence of delta variant’s infectivity or the vaccines’ varying effectiveness.

Based on the data that we have so far, it is a combination of both factors. 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.

Dr. Venky Soundararajan, lead author

The study has not been peer-reviewed yet. It is currently available on the preprint server medRxiv.


Puranik, Arjun, et al. “Comparison of Two Highly-Effective MRNA Vaccines FOR Covid-19 during Periods of Alpha and Delta Variant Prevalence.” 2021, doi:10.1101/2021.08.06.21261707.


Please enter your comment!
Please enter your name here