The infectious period varies but an infected person can probably pass on the disease the day before the symptoms appear and remain potentially infectious for 3-5 days. Routine vaccination offers the best protection and people who are at high risk of infection should be vaccinated. It is difficult to avoid infection if there is an epidemic. Keeping away from crowded places can reduce the risk of becoming infected and spreading it to others. A previous flu infection or vaccination will not necessarily provide protection against further infections because the virus is continually changing genetically and different subtypes circulate each winter.
People develop immunity either as a result of becoming infected, or through vaccination.
And as immunity is specific to individual strains of the virus, the emergence of a new strain will mean that people will contract the disease and a new vaccine will need to be developed. Haemagglutinin is the main antigen associated with immunity. The drifted strains of influenza may infect partially immune people who have been exposed in previous winters. B. Antigenic shift Genetic shift occurs when major changes in the HA or NA take place and a virus emerges which contains a haemagglutinin different from those of previously circulating viruses. Influenza virus strains are classified according to the place and year in which they were first isolated.