Research suggests a single dose of the AstraZeneca is only 33% protective against the Delta variant.
Vaccination is the safest way to gain immunity to the coronavirus, and with most Covid-19 vaccines, you need two doses to get the greatest possible protection. Immunologists call this method of giving people multiple doses “ prime-boosting ”. Essentially, you first teach the immune system what it is looking for – you prime it with the first vaccination. Then, after the immune system has had time to respond to the priming, it is challenged again. The immune response learned the first time is boosted when put into practice on the second exposure. Most (though not all) vaccines – Covid-19 or otherwise – use prime-boosting to maximise protection. Some even require further doses. This is because immunity can wane over time or, as is the case with the flu vaccine, the virus that causes the disease can change so that the original vaccine no longer provides good protection. With Covid-19, we have strong evidence that taking a second dose increases your defence against the disease. Despite this, one in 40 people in the United Kingdom who have had the first dose do not seem to be getting their second. In the United States, there have been reports of millions of people skipping their second doses. In doing so, these people are endangering their health. The benefits of prime-boosting with Covid-19 vaccines were initially seen in clinical trials. Early studies of the currently approved Covid-19 vaccines compared the immune responses in people who received one or two vaccine doses. Results showed that after the second dose, people had higher amounts of antibodies to fight the virus, indicating that taking more than one dose would probably increase vaccine effectiveness. Larger clinical trials then continued with this prime-boosting regimen. With the Pfizer/BioNTech vaccine, protection against symptomatic Covid-19 was found to be 52% from 12 days after the first dose (immunity takes time to build). Protection then rose to 95% after the second dose. With the Oxford/AstraZeneca vaccine, protection against symptomatic Covid-19 from 22 days after the first dose was estimated to be 76%.
