DSeasonal Influenza in Sri Lanka
Seasonal influenza viruses circulate and cause disease in humans every year. In Sri Lanka, for the last few years, it has been generally observed during April to June and again in November to January. It spreads from person-to-person through sneezing, coughing, or touching contaminated surfaces. Seasonal influenza viruses can cause mild to severe illness and even death, particularly in some high-risk individuals. Persons at increased risk for severe disease includes pregnant women, the very young and very old, immune-compromised people, and people with chronic underlying medical conditions such as cancer, diabetes mellitus and heart diseases. Seasonal influenza viruses evolve continuously, which means that people can get infected multiple times throughout their lives. Therefore the components of seasonal influenza vaccines are generally reviewed biannually and updated periodically to ensure continued effectiveness of the vaccines. Recently, the National Drug Regulatory Authority of Sri Lanka has given license for the use of seasonal influenza (flu) vaccine.
There are three large groupings or types of seasonal influenza viruses, labeled A, B, and C. Type A influenza viruses are further divided into subtypes according to the specific variety and combinations of two proteins that occur on the surface of the virus, the hemagglutinin or “H” protein and the neuraminidase or “N” protein. Currently, influenza A (H1N1) is the commonly circulating seasonal influenza A virus subtype in Sri Lanka. During the current season, it has been observed to generally cause severe disease. This seasonal A (H1N1) virus is the same virus that caused the influenza pandemic and it first appeared in Sri Lanka in two waves during 2009/ 10 period.
In addition, there are two type B viruses that are also circulating as seasonal influenza viruses, which are named after the areas where they were first identified, Victoria lineage and Yamagata lineage. Type C influenza causes milder infections and is associated with sporadic cases and minor localized outbreaks. As influenza C poses much less of a disease burden than influenza A and B, only the latter two are included in seasonal influenza vaccines.