The source of the outbreak had still not been established and was under investigation. The average age of the 15 who died was 74 to 92 years and the 88 cases was 74 years of age. It was also noted that within 11 cases covid-19 had also been detected as a co-infection.

The first cases was detected in Matosinhos Vila do Conde and Póvoa de Varzim at the end of October 2020. In the following days there was an increased succession of cases that led to the health authorities analysing public water in the areas affected.

Rui Capucho, a doctor at the ARS-N Public Health Department said,

"We established a relationship between the cooling towers of some industries in the region at the end of the outbreak. But, unfortunately, we cannot yet establish a causal link between any of the concrete towers as a source of infection.

When taking samples [of water from the cooling systems] in the Longa Vida tower, a high concentration of the bacteria was detected, but it has not yet been found in the serotype that infected the patients.”

The outbreak was declared ceased on the 13th January 2021.