Yale study details breakthrough COVID cases and who is most at risk

Yale University has published a new study that evaluates who is most at risk of breakthrough COVID-19 infections after being fully vaccinated. Though the infections remain very rare compared to those who aren't vaccinated, the number of cases has increased as variants spread, particularly the highly infectious Delta variant.

In most cases, fully vaccinated individuals who contract a breakthrough case of COVID-19 experience no or very mild symptoms. However, a small percentage of these individuals may go on to develop a serious illness despite the vaccine — and it's these cases the new study looked into.

The findings are based on 969 cases of COVID-19 among patients hospitalized at the Yale New Haven Health System. Of those cases, which took place over a 14-week period earlier this year, 54 of the patients had been fully vaccinated prior to infection. The patients experienced a variety of severity during their infections.

A total of 14 breakthrough cases reached a 'severe' or 'critical' stage, meaning they required supplementary oxygen while hospitalized. Four of these patients ended up in the ICU and three of them died. Of the vaccinated patients who developed severe COVID-19, most were on the older end of the age spectrum, falling between 65 and 95 years old — their median age was 80.5.

Preexisting comorbidities were also present, including type-2 diabetes and heart disease. Some of the severely impacted patients were also taking immunosuppressive drugs, which can reduce how effective the vaccine is at offering protection.

Hyung Chun, the study's senior author, explained:

It's clear that the vaccines are highly effective, and without them we would be facing a much deadlier pandemic. As effective as the vaccines are, with emerging variants and increasing cases of breakthrough infections, we need to continue to be vigilant in taking measures such as indoor masking and social distancing.