The Delta variant can evade antibodies that target certain parts of the virus, according to a new study published on Thursday in Nature. The findings provide an explanation for diminished effectiveness of the vaccines against Delta, compared with other variants. The variant, first identified in India, is believed to be about 60% more contagious than Alpha, the strain that was first detected in the UK, and perhaps twice as contagious as the original coronavirus.

 

The new study found that Delta was “barely sensitive” to one dose of vaccine, confirming previous research that suggested that the variant can partly evade the immune system — although to a lesser degree than Beta, the variant first identified in South Africa.

French researchers tested how well antibodies produced by natural infection and by vaccines neutralize the Alpha, Beta and Delta variants, as well as the original version of the virus. The researchers looked at blood samples from 103 people who had been infected with Covid. Delta was much less sensitive than Alpha to samples from unvaccinated people in this group, the study found. One dose significantly boosted the sensitivity, suggesting that people who have recovered from Covid still need to be vaccinated to fend off some variants.

The team also analyzed samples from 59 people after they had received the first and second doses of the AstraZeneca or Pfizer-BioNTech vaccines. Blood samples from 10% of people immunized with one dose of the Astra or the Pfizer shots were able to neutralize Delta and Beta in lab tests. But a second dose boosted that number to 95%. There was no major difference in the levels of antibodies elicited by the two vaccines. “A single dose of Pfizer or AZ was either poorly or not at all efficient against Beta and Delta,” the study said.