The increased number of cases of infection by the influenza virus in the last trimester of 2021 and on the first days of 2022 has drawn attention to an old companion of humankind. The flu, as it is popularly known, has been generatiing regional outbreaks all over the country, boosted by the introduction of a new strain of subtype A(H3N2), the Darwin strain. The new strain was first identified in Brazil by the Laboratory of Measles and Respiratory Viruses of the Oswaldo Cruz Institute (IOC/Fiocruz), in samples from the city of Rio de Janeiro.

There are currently three known types of influenza virus: A, B and C. The first two mainly cause seasonal epidemics in different parts of the world, while the last usually causes milder cases. Type A is classified in subtypes, such as A(H1N1) and A(H3N2). Type B is divided in two lineages: Victoria and Yamagata.

Despite theier genetic differences, all types can cause similar symptoms, such as high fever, cough, sore throat, headache, aching body and joints, shivering, and fatigue.

The Drawin strain, recently discovered in Australia, is part of the A(H3N2) type. In the past few months, it has contributed to an increase in the number of cases of influenza in an atypical period in Brazil, which, like other countries of the southern hemisphere, normally sees a higher circulation of the influenza virus in the winter months  (between July and September).

According to Fernando Motta, researcher of the Laboratory of Measles and Respiratory Viruses of the IOC, the large number of people infected with the flu virus is also the result of a combination ofa reduced circulation of the influenza virus in 2020 and the low adhesion to the vaccination campaign this year.

The researcher mentions that the measures to avoid contagion and transmission of the flu are the same ones adopted against Covid-19. “Social distancing, avoiding crowded situations, wearing masks, frequent hand sanitizing, and respiratory etiquette. These are measures we saw in the last two years and thanks to which many respiratory viruses probably stopped circulating. These measures certainly mitigated coronavirus transmission too”, says Motta.