The longer it has been since you were vaccinated, the more likely you are to get infected. A third booster shot is therefore probably not a luxury.

People who have received both doses of the Pfizer vaccine are increasingly likely to become infected from three months after the second shot. This is according to Israeli research. The results confirm that the protection of the corona vaccine decreases over time. And so a booster shot is probably not an unnecessary luxury.

Israel
The study was conducted by the Leumit Health Services Research Institute in Israel. Israel was one of the first countries in the world to roll out a large-scale COVID-19 vaccination campaign last December. In the meantime, however, the infections are also increasing in this country. Since last June, there has been a revival in the number of corona infections in particular. And not only there. Around the world, large-scale vaccination campaigns have kept the spread of the virus under control for some time. But even in countries with high vaccination rates, breakthrough infections are currently reoccurring.

Decrease protection

Scientists suspect that these breakthrough infections are due to a gradual decline in the protection of the corona vaccine. To study this further, scientists examined the results of corona tests of more than 80,000 adults vaccinated with Pfizer. The team found that the number of positive test results increased the more time elapsed after the second shot. For example, only 1.3 percent of participants who had received their second dose between 21 and 89 days ago tested positive. This percentage then increased to 2.4 percent after 90-119 days, 4.6 percent after 120-149 days, 10.3 percent after 150-179 days, and 15.5 percent after 180 days or more.

Infection risk
Compared to the first 90 days after the second dose, the infection risk was 2.37 times higher after 90-119 days; 2.66 times higher after 120-149 days; 2.82 times higher after 150-179 days and 2.82 times higher after 180 days or more.

According to the researchers, the results indicate that the risk of corona infection starts to increase slowly but surely about 90 days after the second corona shot. This does not alter the fact that the Pfizer vaccine offered excellent protection in the first weeks after vaccination, the researchers emphasize. However, the findings do mean that the vaccine’s protection for some individuals diminishes over time.

Limits

The researchers acknowledge some limitations of their study. For example, they cannot rule out that other unmeasured factors – such as household size, population density or virus strain – may also have influenced the results. However, the study was conducted on a large number of people who all received the same vaccine. This allowed the researchers to conduct a detailed analysis, suggesting that the results are robust.

All in all, the researchers state that a booster shot may not be an unnecessary luxury. This booster vaccination is intended as a boost that can improve the effect of the first vaccination series. The future will then have to show whether such a third injection is sufficient or whether it is necessary to administer the booster regularly – for example annually.