Although the flu seems like a harmless disease, it can lead to complications and even death, especially in people at high risk. That is why prevention is so important. Nowadays, the vaccine keeps the virus from circulating and reduces the number of hospitalizations, especially during activity peaks¹.

There are four types of influenza/flu viruses (A, B, C and D)¹:

“Type A” are subdivided based on the combination of two proteins: Hemagglutinin (HA) and Neuraminidase (NA). Among the influenza virus A subtypes, A(H1N1) and A(H3N2) circulate seasonally and infect humans1.

“Type B” are divided into to primary groups (lineages), named lineage B/ Yamagata and B/ Victoria. Influenza B viruses are not classified into subtypes1.

Only the influenza A viruses are responsible for major pandemics1.

“Type C” are detected much less often and typically cause mild infections, without any relevance for public health1.

Influenza D viruses affect primarily livestock and are not known to infect or cause diseases in humans1.


Risk groups¹

All age groups are susceptible, but some groups are at higher risk than others. People at highest risk of developing the severe form of the disease or complications when infected:

  • Pregnant women;
  • Children under 59 months;
  • Elderly;
  • Individuals with chronic medical conditions (e.g. heart, lung, kidney, metabolic, neurological, liver or blood diseases) and individuals with immunosuppressive conditions (e.g. HIV/AIDS, undergoing chemotherapy or steroids, or developing malignancy);
  • Healthcare professionals (due to higher exposure to patients, running the risk of spreading particularly among vulnerable individuals).



The vaccine is definitively the most effective form of prevention against the flu and its complications. However, constant evolutions of the influenza virus require global and frequent monitoring, which always leads to a reformulation of the vaccine. That is why a new dose must be taken every year.

According to the World Health Organization (WHO), the recommendation regarding the composition of the vaccine (trivalent) targets the three most representatives types of viruses in circulation (two subtypes of influenza A and one influenza B virus).

It is important to note that the flu vaccine is ineffective against Covid-19 (coronavirus), the new virus that causes the same clinical symptoms as influenza².


1 – Influenza (seasonal) – World Health Organization. Available at Last accessed on March 27, 2020.
2 – Q&A: Similarities and differences – World Health Organization. Available at Last accessed on March 27, 2020.