Influenza immunity, or biological resistance to the influenza virus, can be acquired in several different ways, such as vaccination or past exposure to the virus. The influenza virus is a contagious respiratory infection that can cause severe respiratory symptoms and can lead to complications that may be fatal. It is difficult to fabricate effective vaccines that cause influenza immunity because the virus rapidly changes through an almost-constant series of mutations. Yearly flu outbreaks occur around the world and cause the deaths of hundreds of thousands of people each year. New, particularly deadly strains of the influenza virus generally acquired from different animals such as pigs or birds can lead to even more deaths.
Partial influenza immunity can be acquired simply through exposure to the influenza virus. If an individual catches the virus one year, he is considerably less likely to catch it the following year, as his system has developed some level of resistance to the virus. This is only true, however, because influenza viruses tend not to mutate drastically in the space of a year. If a new strain of the virus does appear, an individual exposed to a previous strain of the virus probably will not have the same level of influenza immunity. Strains similar to those caught in previous years can still cause infection as well, particularly if the infected individual is already in poor health.
Each year, scientists make vaccines to offer influenza immunity to those who need it. There are two commonly-used types of vaccines. In one, deactivated, harmless influenza agents are injected through a needle into the arm. The other vaccine, known as the live attenuated influenza vaccine (LAIV), is made with living but weakened flu viruses and is administered as a nasal spray. Both vaccines can result in influenza immunity that protects against the most prevalent strains of the virus in a given year.
While the influenza immunity that results from vaccination does save many lives each year, the vaccines can also cause harmful or even deadly side effects and complications. This is particularly true for people in poor health. The influenza viruses in the nasal spray, though too weak to overcome a healthy immune system, could still infect and harm an individual with a compromised immune system. Side effects of the vaccines can include fever, soreness around the area where the shot was given, and flu-like symptoms. Healthy people generally overcome these side effects quickly, and some may not experience them at all.
