Helpful delay: The protective effect of the AstraZeneca corona vaccine can be improved by increasing the interval between the two vaccine doses. This is suggested by new evaluations of the study data. According to this, the effectiveness increases to 81 percent if the two vaccine doses are administered twelve weeks apart. If the gap is less than six weeks, it is only a good 55 percent, as researchers report in the specialist magazine “ The Lancet”.
The corona vaccine from AstraZeneca and the University of Oxford is controversial in this country. Study data, for example, confirm that it is only 60 to 70 percent effective against Covid-19 – significantly less than the mRNA vaccines or the Russian Sputnik V vaccine. In addition, reports of side effects and insufficient effectiveness against some mutants of SARS-CoV-2 cause uncertainty. Last but not least, AstraZeneca received heavy criticism for its preferred delivery of the vaccine to the UK.
Should the second dose be postponed?
Regardless of the type of vaccine, however, there is also disagreement as to whether it is sensible and permissible to lengthen the time interval between the two vaccine doses. According to the approval, there should be three to six weeks between doses, depending on the vaccine. However, due to the shortage of vaccines, Great Britain, among others, has decided to extend the interval. In this way, more people can receive at least a first vaccination despite the shortage of supplies.
A new analysis of study data now provides indications that such a delay in the second dose could also be medically advantageous – at least for the AstraZeneca vaccine. To this end, Merryn Voysey from the University of Oxford and her colleagues compared the data from 17,178 test subjects in Great Britain, Brazil and South Africa who received their two vaccine doses at different intervals or who were vaccinated with only one dose. Half had received a placebo, the other half received the real vaccine, which is also known technically as ChAdOx1.
Clear protective effect after the first dose
The evaluations showed: Even after the first vaccination dose, the AstraZeneca vaccine protects 76 percent against a symptomatic course of Covid-19. This vaccination protection apparently remains in place for at least three months: “During this time, there were no signs of a decrease in the protective effect,” the researchers report. The number of antibodies against SARS-CoV-2 also decreased only insignificantly in the test subjects in the three months.
According to the scientists, this suggests that the first vaccination already provides reasonable protection: “If there is a shortage of vaccines, then it might be better for the protection of the population to first give as many people as possible an initial dose”, says study leader Andrew Pollard from the University of Oxford. The second dose is necessary to provide long-lasting immune protection.
Dose delay increases effectiveness to 81 percent
A second result is even more interesting: if the interval between the two vaccine doses is extended, the protective effect of the vaccine increases. Specifically, the effectiveness of the AstraZeneca vaccine was only 55.1 percent with a dose interval of six weeks or less. However, if there were twelve weeks in between, the vaccine protected 81 percent from Covid-19, as the scientists report.
This observation is supported by immunological tests. Subjects who received their vaccination doses twelve or more weeks apart had twice as high antibody titers in their blood as those with the shorter vaccination interval. As the researchers explain, these observations fit with the results of other vaccines against Ebola, malaria or influenza, for which a longer dose interval also increases the immune response and protective effect.
What’s the better strategy?
This positive delay effect could possibly also clear up the confusing results of the clinical studies with regard to the level of the dose. Because some of the test subjects had only received half the amount of vaccine for the first dose – and that, oddly enough, increased the effectiveness. “Now we have found that the increased effectiveness could be partly due to the longer dose intervals in this group of subjects,” says Voysey.
Even if the clinical studies were not originally designed to determine the optimal vaccination interval, the researchers believe that the current evaluation provides important information. “Vaccination programs in which a large part of the population first receives one dose and the second only three months later, could be an effective strategy against the pandemic,” state Voysey and her team. ( The Lancet, 2021; doi: 10.1016 / S0140-6736 (21) 00432-3)
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AstraZeneca vaccine longer interval works higher protective effect vaccine dose twelve weeks