Surveillance and outbreak alert

Seasonal Influenza (H1N1)

WHO/Tom Pietrasik

Seasonal influenza viruses circulate and cause disease in humans every year. In temperate climates, disease tends to occur seasonally in the winter months, spreading 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 include pregnant women, the very young and very old, immune-compromised people, and people with chronic underlying medical conditions. 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 reviewed frequently (currently biannually) and updated periodically to ensure continued effectiveness of the vaccines.

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) and A(H3N2) are the circulating seasonal influenza A virus subtypes. This seasonal A(H1N1) virus is the same virus that caused the 2009 influenza pandemic, as it is now circulating seasonally. 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.

Do's and don'ts to protect from seasonal flu

Do...
- wash your hands
- stay more than one arm's length distance from persons sick with flu
- avoid crowded places
- eat nutritious food and drink plenty of water
- get plenty of rest

Do not...
- shake hands or hug
- spit in public
- take medicines without consulting a physician

If you are sick, please
- cover your cough or sneeze
- stay home and limit contact with others as much as possible
- rest and take plenty of liquids
- seek medical advice

Contact us

Disease Surveillance and Epidemiology
World Health Organization
South-East Asia Regional Office
Indraprastha Estate, Ring Road, New Delhi-110 002
Tel: +91 11 23370804
Fax: +91 11 23705663 (Direct)
Email: outbreak@searo.who.int