Covid-19: Vaccine effectiveness against Delta variant wanes over time, new study finds

University of Oxford and Office for National Statistics researchers conducted the study, the results of which were published in a pre-print

The effectiveness of Oxford-AstraZeneca and Pfizer-BioNTech vaccines against the Delta variant of the coronavirus wanes over time despite offering considerable protection against the strain, a UK study has found.

University of Oxford and Office for National Statistics researchers conducted the study, the results of which were published in a pre-print.

The researchers analysed results of 2,580,021 PCR tests collected from 3,84,543 people from December 1 and May 16, 2020, and another 8,11,624 results from 3,58,983 people taken between May 17 and August 1.

Key findings
The study found that both AstraZeneca and Pfizer vaccines offered considerable protection against the highly contagious Delta variant, but the effectiveness reduced over time.

The researchers found that the Pfizer vaccine offered greater protection initially, but its effectiveness also declined faster. Moreover, two doses of both the vaccines provide at least the same protection acquired through natural infection. The study also found that people who take the jab after being infected have greater protection.

A single dose of Modern’s Covid-19 vaccine offered greater or similar effectiveness against the mutation as a single dose of other available vaccines, the study found.

Waning effectiveness
The Pfizer vaccine offered 92 per cent protection from high viral load 14 days after the second dose against AstraZeneca’s 69 per cent, the study found. The Pfizer vaccine’s effectiveness dropped to 90 per cent in case of infections with a higher viral load a month after the second dose. It declined further to 85 per cent two months after the second dose and to 78 per cent after three.

The equivalent protection offered by the AstraZeneca vaccine was at 67, 65, and 61 per cent, respectively.

Breakthrough infections
The study shows the Delta variant can infect vaccinated individuals.

Delta variant infections also led to peak viral loads, similar to those found in unvaccinated individuals, even in vaccinated people. The peak load in Alpha-variant infections after vaccination was found to be much lower.

Sarah Walker, chief study investigator and a medical statistics professor at Oxford, said it was still unclear how much transmission occurred from people infected with the Delta variant post vaccination.

The high virus levels suggest that unvaccinated people may not have as much protection from the Delta variant as earlier hoped, she said. This means it has become essential to vaccinate as many people as possible.

The study, however, did not indicate how effective vaccines were in preventing severe infections and hospitalisation.

Technion-Israel Institute of Technology Biomedical Data Scientist Dvir Aran told Nature that the UK study could explain Israel’s high breakthrough infections.

Aran said Israel was witnessing high breakthrough infections among individuals vaccinated early, while those vaccinated recently, particularly 12-15-year-olds, were protected.

Three recent studies by the US Centers for Disease Control and Prevention (CDC) have also found reduced vaccine effectiveness over time, leaving people more vulnerable to breakthrough cases. These results will lead to more debates over the need for booster doses over time.

Vaccine interval and effectiveness
The UK study found no relation between the vaccines’ effectiveness and the gap at which the doses were administered.

Nuffield Department of Population Health Senior Researcher Dr. Koen Pouwels told the University of Oxford website there was no effect of the interval on effectiveness. He added the greater effectiveness of two doses supported the decision to cut the interval to eight weeks.

The study also found vaccines offered more protection to younger people than older individuals.