A study published Thursday (8 July) in the journal Nature found that a single dose of the Pfizer or AstraZeneca vaccines—both of which require two injections—was only marginally or not at all effective against the Delta variant.
The researchers conducted laboratory experiments on blood samples from people who had received one of these injections. After a single dose, only 10% of these samples developed antibodies that neutralised the Delta variant—a sign that these people would be protected against symptomatic infection.
After two doses, however, 95% of the samples had developed antibodies that neutralised the Delta variant. The researchers concluded that the Delta variant 'partially but significantly evades' the immune protection of vaccines.
96% effectiveness against severe forms
While the results of laboratory experiments such as these are not necessarily directly transferable to the real world, other studies have similarly shown that Delta is resistant to the protection of one dose of vaccine.
A UK analysis in May showed that a single dose of Pfizer or AstraZeneca's vaccine was only 33% effective against symptomatic COVID-19 caused by Delta. After two doses, the efficacy increased to 88% for Pfizer's vaccine and 60% for AstraZeneca's vaccine. Two doses of Pfizer's vaccine were also 96% effective in preventing hospital admissions due to Delta, while two doses of AstraZeneca's vaccine were about 92% effective by the same criterion.
Meanwhile, a Canadian study awaiting peer review found that a single dose of Pfizer's vaccine was 56% effective in preventing symptomatic infections caused by Delta after two weeks. This compares to 67% for AstraZeneca's vaccine and 72% for Moderna's vaccine. For the prevention of Delta-related hospital admissions, the efficacy increased to 78% for Pfizer, 88% for AstraZeneca and 96% for Moderna.
The same study suggests that after two doses, Pfizer's vaccine was 87% effective against symptomatic infections caused by Delta. But the researchers did not have sufficient data for AstraZeneca or Moderna.
Many people stop one dose
Taken together, the studies suggest that partially vaccinated people may be more vulnerable to symptomatic cases of COVID-19 now than in past months, as Delta has become the dominant strain in many countries, including the UK and the US. About 48% of the US population is fully vaccinated, while 55% has received at least one dose.
But in some states, the gap between those who have received a single dose and those who have completed the course is larger than the national average. Arkansas state health director Dr Jose Romero told Insider that 15% of people who received their first dose of Pfizer or Moderna vaccine did not return for their second dose a fortnight ago. In many cases, José Romero explained, people were put off by the initial side effects.
We have a significant proportion of people who get one dose of a two-dose series but don't come back for the second dose within the window, or within 42 days after that vaccine.
They don't have the full protection they should have.