COVID-19 vaccine efficacy wanes under delta variant, study shows

As the highly contagious delta variant continues to spread, a new, British study shows COVID-19 vaccines are effective against the delta variant, but that their protection decreases over time, weakening significantly within three months.

According to the pre-print study, investigators with Oxford University and the United Kingdom’s Office for National Statistics found that those who get infected with COVID-19 after receiving two shots of either the Pfizer-BioNTech or the AstraZeneca vaccine may be of greater risk to others than with previous variants of the coronavirus.

The investigators looked at data from more than 2.58 million swab PCR tests taken between Dec. 2020 and May 2021 — when the original wild type variant was dominant — and more than 811,000 test results between May 2021 and Aug. 2021, when the delta variant was dominant. 

"The Pfizer/BioNTech vaccine was 92% effective in keeping people from getting a high viral load, however, the effectiveness waned to 85% after 60 days and 78% after 90 days. Meanwhile, the Oxford/AstraZeneca vaccine was 69% effective against a high viral load 14 days after the second dose. That dropped to 61% after 90 days," the university wrote in a press release. 

RELATED: Record pediatric COVID-19 hospitalizations reported amid delta surge

Two doses of either vaccine still offered protection, signaling that two vaccine doses remain the most effective way to ensure protection against the delta variant. 

However, researchers found delta infections after two vaccine doses had similar peak levels of virus to those in unvaccinated people. With the Alpha variant, peak virus levels in those infected post-vaccination were much lower.

Professor Sarah Walker, the chief investigator and academic lead for the COVID-19 infection study said, "We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time. But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped. This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide." 

The 39-page study results suggested that after four to five months, effectiveness of these two vaccines would be similar – however, long-term effects need to be studied.

RELATED: Study: Moderna COVID-19 vaccine protects against delta variant up to 6 months

Moderna announced on Aug. 12 a recently published study found their COVID-19 vaccine was 93% effective against the novel coronavirus and its variants, including the highly contagious delta variant, for up to six months, according to a company news release, suggesting continued protection for a longer length of time.

The study, published in the journal Science noted that antibodies created by Moderna’s COVID-19 vaccine were able to provide continued protection against the virus six months after patients received their second dose.

"We are pleased with these new data showing that people vaccinated with two doses of the Moderna COVID-19 vaccine maintained antibodies through six months, including against variants of concern such as the Delta variant. Along with our partners, we are committed to generating data on the Moderna COVID-19 vaccine and sharing this as available," said Stéphane Bancel, chief executive officer of Moderna.

This study comes as COVID-19 infections surge across the U.S. For the first time since the COVID-19 winter surge, coronavirus hospitalizations have topped more than 100,000, according to the U.S. Department of Health and Human Services.

As of Aug. 25, the department’s online dashboard reported 100,317 impatient beds are in use for COVID-19 with more than 5,400 hospitals reporting the data.