A new study found that two key COVID-19 vaccines in the United States have seen their effectiveness weakening as the highly contagious Delta variant is spreading wider.

The study from Massachusetts-based data analytics company nference, released on Sunday, Aug. 8, showed that the efficacy of Moderna’s vaccine dropped from 86% in January 2021 to 76% in July.

Meanwhile, Pfizer’s vaccine effectiveness declined at a sharper rate, from 76% early this year to 42% in July, The Hill reported.

The results were revealed after nference researchers had sought to find one vaccine that may be superior against the Delta variant of COVID-19.

The researchers compared the effectiveness of the Moderna and Pfizer-BioNTech vaccines from January through July by studying more than 50,000 patients in the Mayo Clinic Health System.

According to the study, that has yet to be peer-reviewed, all COVID-19 vaccines being administered have shown to dramatically reduce the risk of infection, severe disease and death, but Moderna’s vaccine may provide more protection than Pfizer’s when it comes to the Delta variant.

However, their efficacy began to wane in July when Delta became the dominant strain of COVID-19 in the United States. 

Pfizer itself has said a third dose of vaccine may be needed within 6 to 12 months after full vaccination.

Moderna, earlier this month, advised the American people to take a third shot of the vaccine against COVID-19 to protect against future mutations of the coronavirus.

In clinical trials, Pfizer claimed that its vaccine showed to be 95% effective in preventing symptomatic COVID-19, while Moderna said its vaccine was 94% effective.