Vaccines against COVID-19 that were developed by US drug makers Pfizer and Moderna may not be as effective against the 'Delta' variant compared to as they were against the original strain of the virus, suggests a new study.

For our comprehensive coverage and latest updates on COVID-19 click here.

Researchers from the Mayo Clinic in Minnesota, US, found that the Pfizer-BioNTech vaccine was only 42 per cent effective against infection in July, while the Moderna vaccine was only 76 per cent effective, the Daily Mail reported. "Comparing rates of infection between matched individuals fully vaccinated with mRNA-1273 (Moderna COVID vaccine) versus BNT162b2 (Pfizer COVID vaccine) 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a two-fold risk reduction against breakthrough infection compared to BNT162b2," the researchers wrote in the study, not yet peer-reviewed, and published on pre-printer server medRxiv.org. For the study, the team gathered data on more than 25,000 Minnesotans from January to July.

From January to June, the vaccines as claimed remained effective around 90 per cent, but began dipping in June and largely dropped in July as the variant took hold in the US. The change in vaccine effectiveness corresponds with a massive surge in the prevalence of the Delta variant in Minnesota, growing from 0.7 per cent prevalence in May to more than 70 per cent in July, the study showed. Meanwhile, the 'Alpha' variant, the previous dominant strain in the US, decreased in prevalence from 85 per cent to 13 per cent over the same time period.

The US is currently seeing a spike in infections and death due to the Delta variant. As of Thursday morning, the overall caseload and death toll in the US stood at 36,185,761 and 618,454, respectively, according to Johns Hopkins University. It is projected to increase in the coming weeks.

However, the vaccines are still effective in preventing hospitalisations and severe cases from the virus, with both having a hospitalisation rate of under 25 per cent, the researchers said. Last month, Pfizer published data that showed that its vaccine's efficacy drops to 86 per cent after six months. But booster shots, expected to be rolled out soon, can help enhance immunity to the virus and protect against more resistant variants.