In the summer of 2021, researchers took samples of nasal mucus from more than 600 people who worked or lived in three residential care centers in Nivelles, Liège and Zaventem. Large outbreaks of COVID-19 have occurred in both care homes, with more than a fifth of residents who eventually test positive for the virus dying from the disease. Almost the entire population has been vaccinated twice.

The striking difference in the three outbreaks was that they involved three different types of coronavirus. In one center the delta variant was involved, another center had to deal with infection with the gamma variant, and a third center had the mu variant, which is less known to the general public. The latter, the mu variant, has not been seen as a “worrying halo variant”, but an “interesting variant”.

“It was a surprise that the ‘non-worrying’ mu variant can also lead to many deaths,” says immunologist Johan van Wenberg (Riga Institute-KU Leuven). “That’s why we wanted to see how this might be explained, and what the three outbreaks had in common.”

Analyzes of nasal swabs showed that the coronavirus was more likely to cause a fatal outcome in people with a high amount of a specific immune system cytokine protein. “The protein triggers an inflammatory response within the immune system of someone infected with the virus,” Van Weyenbergh explains. “The more cytokine protein found in people’s nasal swabs, the faster they died.”

The research also revealed that the virus had been in the air for a very long time. Aerosols containing viral particles were still present in residential care centers for more than fifty days after the first case was detected. “This points to the importance of good ventilation,” says the immunologist. Another risk factor was the high viral load in the noses of the population. New vaccines could target this: by generating immunity where the virus hits first, namely in the nose and throat, people can be better protected against serious disease and infection.”