Immune system, cyclical influenza incidence

Two issues, both related to the human body’s immune system, need to be understood in relation to the cyclical incidence of influenza. One bout of infection with the influenza virus confers immunity to the infected person as the body develops specific antibodies to fight against the infection. The period for which such immunity lasts is variable, depending on the person’s age and general immune status. The immunity may last for shorter periods in older people and people with an already compromised immune system (for example those who are suffering from serious chronic illnesses and those taking some forms of medication). The immunity is to the specific infecting influenza virus though individuals usually acquire partial immunity to other variants of the influenza virus as well. When a new viral strain starts infecting people, it is always much more infective as no one has been previously exposed to the particular virus. Over a period, as more people are infected and thus become immune for variable periods, the infection rate of the virus starts declining. Thus, both in 1918 and 2009, the number of infected people declined dramatically over two years. The working of the immune system also explains why there are periodic spikes in seasonal influenza cases, every few years. Every season there are a number of influenza cases reported, but every 3-4 years there is a perceptible rise in cases in a particular year. This happens because when the immune levels of a large number of people starts declining, which means the protective effect of previous infections start waning, a larger number of people get infected in a particular season. What we are seeing in India now is a rise in H1N1 influenza cases, as the protective effect on the population of the global pandemic of 2009 has receded. It is important to remember that seasonal influenza is caused by a number of variants of the influenza virus. It is quite likely that the H1N1 virus has become, at least in some parts of the country, the major variant that causes seasonal influenza.