Oxford study: Vaccinated with Delta similarly contagious as unvaccinated

Two vaccinations with Biontech / Pfizer or AstraZeneca offer good protection against the highly contagious Delta-Corona variant. But the effectiveness decreases over time – and an infection with the Delta variant triggers a viral load similar to that of non-vaccinated people in fully vaccinated people, scientists from the University of Oxford found in a study published on Thursday. If they become infected, people who have been vaccinated may be as contagious as those who have not been vaccinated.

“You are still less likely to get infected if you’ve been given two doses. But if you do, you’ll have a virus concentration similar to someone who hasn’t been vaccinated at all,” said Sarah Walker, professor of medical statistics and epidemiology at the Oxford University who directed the study.

In the alpha variant, the viral load in people who became infected despite a vaccination was significantly lower. “The fact that we are seeing higher viral loads suggests that herd immunity may indeed become more difficult,” said Oxford scientist Koen Pouwels. “Vaccines are probably best used to prevent serious illness and a little less to prevent transmission.”

For the study, which has not yet been peer-reviewed, the researchers analyzed results from more than three million adult nasal and throat swabs in the UK. To compare the time periods before and after Delta spread, the researchers analyzed about 2.58 million smears from around 380,000 adults between early December and mid-May, and 810,000 test results from 360,000 participants between mid-May and early August.

They also found that protection from infection 90 days after the second vaccination was 75 percent with Biontech / Pfizer and 61 percent with AstraZeneca. Two weeks after the second dose, this was still 85 percent and 68 percent, respectively. “These data tell us nothing about the level of protection against serious illnesses and hospitalization, two very important factors when looking at the effectiveness of the vaccines,” said Pouwels. The researchers did not predict how much protection might decrease over time, but suggested that the effectiveness of both vaccines should be similar four to five months after the second vaccination.