The first large-scale real-world study of the effectiveness of booster shots against COVID-19 has found them to be up to 93 percent protective, even against the Delta variant. There remains considerable debate about both the ethics and wisdom of using scarce vaccination supplies to give people in wealthy countries third doses while billions are unvaccinated elsewhere. Nevertheless, as much as any one study can, the findings put to rest the question of whether Delta can be tamed using existing mRNA vaccines, rather than requiring new versions specifically targeted towards it.
Israel convinced Pfizer to sell it the vaccine doses it needed long before other countries by pointing out its status as a perfect testing ground. This helped it control a winter outbreak, temporarily bringing infections down to negligible levels. However, by July Delta was spreading widely. Infections subsequently set national records, although death rates were less than half the previous worst.
Israel tried the same approach again, starting booster shots to the already fully vaccinated on July 13, long before anyone else, and collecting abundant data. The Lancet has now published an analysis mining this rich vein of information. Although raw figures already suggested booster shots work, the paper controls for confounding factors such as pre-existing conditions.
The study is based on the period from July 30 to September 23 this year, coinciding with most of Israel's fourth wave, which peaked in early September with around 10,000 cases and 30 deaths a day. Almost all the infections were from the Delta variant.
The authors compared 728,321 people who received a third dose of the Pfizer vaccine with an identical number of people who had their second Pfizer injection at least five months earlier, but had yet to receive a booster. The second group were carefully chosen to match the first as closely as possible, on characteristics including age, sex, location, health status, and known risk-taking and risk-avoiding behavior.
The most dramatic finding was that more than a week after getting the booster shot a person's risk of hospitalization for COVID-19 was 93 percent lower than someone with similar characteristics, but only two shots. The booster was almost as protective for being classified as having severe COVID at 92 percent, and for testing positive for COVID-19 (88 percent) and having any noticeable symptoms (91 percent). The results are similar to a previous study that did not control for pre-existing conditions.
Protection against death, at 81 percent, at first looks disappointing by comparison. However, with only seven deaths among the boosted the error bars are large here, unlike for hospitalizations.
The study reports booster effectiveness was consistent across demographics, including between those over and under 70 years old.
"These results show convincingly that the third dose of the vaccine is highly effective against severe COVID-19-related outcomes in different age groups and population subgroups, one week after the third dose,” said senior author Professor Ran Balicer of the Clalit Research Institute in a statement.
The big question the study cannot answer is whether protection from the booster is more enduring than from the original two doses, which is known to wane after six months.
The paper acknowledges but does not engage with the issue that while wealthy nations provide boosters to their own people, COVID-19's unchecked spread elsewhere increases the prospects for even worse variants to emerge. On the other hand, it proves that if distributed widely and rapidly enough, vaccines can bring deaths and hospitalizations from Delta under control.
Publication coincides with evidence mRNA vaccines are five times as protective as previous infection with COVID-19.