One shot of the AstraZeneca or Pfizer vaccine reduces coronavirus infections by nearly two-thirds and protects older and more vulnerable people just as well as younger, healthy individuals, a new study finds. But the most important finding may be that after three weeks, both vaccines offer the same protection.
With both the AstraZeneca and the Pfizer vaccines, the chance of a corona infection decreases by 65 percent from three weeks after the first injection. It is striking that the percentage is about the same for both vaccines: 64 percent with AstraZeneca and 67 percent with Pfizer. This is evident from a study of more than 350,000 Britons who received one of the two vaccines.
This is not only a boost to AstraZeneca’s reputation, it is also very good news for developing countries. They will never be able to afford the Pfizer vaccine, and logistically it is difficult because it has to be stored very cold, unlike AstraZeneca.
First study showing effect of vaccines in the ‘real world’
The results of the study from Oxford University and the Office for National Statistics are the first to demonstrate the impact on new infections and immune responses in a large group of adults in the general population for the two vaccines.
By reducing the number of infections, the vaccines will not only prevent hospitalizations and deaths, but also help break chains of transmission and thus reduce the risk of a damaging resurgence of the disease.
The researchers analyzed Covid test results from more than 350,000 people in the UK between December and April. They found that 21 days after an initial shot – the time it takes for the immune system to build up a decent response – new Covid infections decreased by 65 percent.
The vaccines were more effective against symptomatic than against asymptomatic infections and reduced the rates by 72 percent and 57 percent, respectively, compared to those seen in the unvaccinated population.
A second injection of the Pfizer vaccine further enhanced protection, reducing symptomatic infections by 90 percent and asymptomatic infections by 70 percent. Because the AstraZeneca vaccine was later rolled out, it is too early to assess the impact of those second doses.
The scientists found no evidence that the vaccines were any less effective in older adults or those with long-term health problems. The results, released in preprint, are especially important as they demonstrate the protective effect of the vaccines in the real world against the highly contagious and more deadly British variety, which was not there in the original clinical trials.
At Pfizer, antibodies recede more quickly
The team also analyzed antibody responses to the vaccines. Antibodies rose faster and to a higher level with one injection of the Pfizer vaccine, but then fell back, especially in older people, to a level similar to that achieved with the AstraZeneca injection. Although immune responses differed, a strong antibody response was achieved in 95 percent of people.
In younger people, one injection of the vaccine increased antibodies to levels seen after infection with Covid, while two doses were needed in older people. This underlines the importance of a second dose for better protection.
However, the results also show that after one shot you can still get infected with the virus and it cannot be ruled out that you can also pass it on. The people who did test positive after vaccination, on average, carried less virus.