COVID-19 vaccines work: not misreading the data can make you think otherwise

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Israel data show that the COVID-19 vaccine works, but only if properly analyzed: the unfortunate history of the base rate fallacy.

The proliferation of false information about COVID-19 and vaccines has been a significant risk factor for more disease and mortality throughout the pandemic. As highly vaccinated countries like Israel have experienced an increase in cases due to the delta variant, the viral misinformation on the efficacy of COVID-19 vaccines has also increased.

Israel had one of the first and most successful vaccine launches against COVID-19. In just four months, between January and April this year, they achieved 57% coverage of their total population with the Pfizer vaccine, which correlates with a 98% decrease in COVID-19 incidence rates. during the same period. This first and positive example gave the rest of the world a lot of optimism because we would see the end of the pandemic. Unfortunately, the delta variant (associated with higher transmissibility and higher viral load) emerged as a wave in Israel in mid-June, causing an increase in new cases of COVID-19, including those of fully vaccinated individuals. This phenomenon led vaccine skeptics to promote the idea on social media and elsewhere that the Pfizer vaccine did not work against this more formidable new opponent.

No vaccine is 100% effective and we hope there are advanced cases. The efficacy of the vaccine against COVID-19 infections of the asymptomatic and mild delta variant is slightly lower than that of the alpha variant and the original virus. Protection against vaccines against serious diseases is still strong.

Vaccines work, but just seeing the number of cases in Israel will not prove it to us.

Comparing numbers of vaccinated versus unvaccinated COVID-19 cases in a highly vaccinated population provides a misleading view of vaccine efficacy. This error is known in statistics as a base rate fallacy because the absolute number of cases is not presented in the right context. In a population like Israel, where most eligible people are vaccinated, it is logical that the number of cases is higher in vaccinated people. However, theirs rate of disease will be even lower. Comparing the file rates of COVID-19 between vaccinated and unvaccinated populations gives us a more accurate picture of what is happening.

For example, most cases of severe COVID-19 in Israel occur in people over 60 years of age. As of August 25 (the most recent data available), there were 257 vaccinated people over the age of 60 with severe cases of COVID-19, 13 partially vaccinated, and 208 unvaccinated. Looking at these absolute figures, it seems at first glance that vaccines do not protect against serious diseases in this age group or that it is only better to take a dose. But this is because almost everyone (around 90%) in this age group is completely vaccinated and the other two groups are much smaller.

When we re-analyze the same data from August 25 to take into account the appropriate base rate (cases per 100,000 in the three vaccination groups), we see that the rate of serious cases in the unvaccinated group (266 per 100,000) and partially vaccinated (58 per 100,000) are much higher than those in the fully vaccinated group (19 per 100,000).

This is not only true for severe COVID-19 at older ages. Once the baseline percentage is adjusted, the rates of all COVID-19 infections in Israel are lower in vaccinated people of all age groups.

The base rate fallacy is just one example of widespread misinformation that has caused public confusion about the effectiveness of COVID-19 vaccines. Vaccines work, but viral social media posts that present misleading data can cause others to decide against the vaccine, even though they don’t understand that they are at a higher risk of getting the disease. It is vital that the information used to make health decisions comes from reputable sources and that the data is presented in the right context to draw valid conclusions.

Dr. Katrine Wallace is an epidemiologist and adjunct professor of epidemiology and biostatistics at the University of Illinois at the Chicago School of Public Health. She is also known as the scientific communicator “Dr. Kat ”on her popular social media channels where she educates about the epidemiology and pandemic COVID-19. Follow her Twitter: @drkatepi

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