The Pfizer vaccine, is less than 42% effective against Delta variant infection, according to a new study. The Moderna vaccine, on the other hand, has an efficacy of 76%.
The effectiveness of vaccines against the delta variant appears to be decreasing as studies continue. Already on 4 August, a study in the UK revised the vaccine’s effectiveness against infections down to only 50% (for all vaccines combined). The new study, previously published on the MedRxiv website (and therefore not yet assessed by the Scientific Committee), estimates that the Pfizer-BioNTech vaccine is only 42% effective against infection with the Delta variant, compared to 76% effectiveness with the Alpha variant. The Moderna vaccine appears to be less sensitive to viral mutations, as its efficacy is reduced to “only” 76% against the Delta variant and 86% against the Alpha variant.
60% less likely to be infected with Moderna
The study was carried out on 180 000 people vaccinated between January and July 2021 in Minnesota, at a time when the dominant strain was changing from the Alpha to the Delta variant, allowing a comparison of the two.
Although the efficacy of both vaccines against infection was disappointing, it was still high against the risk of hospitalization – 75% for the Pfizer vaccine and 81% for the Moderna vaccine.
These results are in line with those observed in other states. “In Florida, which is currently experiencing the largest outbreak of Covid-19 to date in the US, the risk of infection in July after complete vaccination with the Moderna vaccine was approximately 60% lower than after a complete vaccination with the Pfizer vaccine,” the researchers stated.
Same technology but some differences
This difference may seem surprising because the two vaccines work on the same principle: injecting messenger RNA, which contains the sequence of the virus’ spike protein and triggers its production in cells. The authors of the study do not provide an explanation for this difference in efficacy and call for further research into it. However, there are some differences between the two types. The Moderna vaccine contains 100 micrograms of mRNA, which is three times more than the Pfizer vaccine (30 micrograms per dose). In addition, the Moderna vaccine is injected pure, unlike the Pfizer vaccine which has to be diluted with 0,9 % sodium chloride. Finally, the composition of the lipid particle containing the mRNA is not the same, which may affect the correct retention of the mRNA.
However, this study, carried out under real conditions, may be affected by some biases, such as the fact that the comparison with unvaccinated individuals is based on an untested population, which may therefore have been previously infected and thus immune. However, it confirms the very high infectivity of the Delta variant, which undermines the strategy of eradicating the virus by vaccination. However, vaccination is still indispensable as it prevents the influx of patients into hospitals and, above all, many deaths, which remains the main objective.
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