Just when you thought you could get the flu and then be "immune" to it for life, you find out that you can get a more severe (2nd generation) version of the same virus, making you sicker this time; then, you worry that the third time you get it (3rd genration) it could kill you.
If you pass the 2nd or 3rd generation version to someone who has not been infected by the virus yet, it could kill them.
"I
came down with the flu in August, it was mild, 5 days of relative
discomfort but the classic symptoms of h1n1 ie., dropping from head
directly into lungs. This past week another bout with the same flu only
it is triple in it’s effects and has really laid me low. I haven’t had
a cold or flu for over 30 years and this really really sucks. I hope
that there isn’t a tertiary infection because anything stronger than
what I have now will land me in the hospital."(shocked and awed Says: November 25th, 2009 at 7:16 am)
When exposed to generation 2 (v 1.2) or 3 (v 1.2) H1N1 viruses, previously health young adults develop more serious symptoms, leading to an increase in hospitalizations and deaths. The higher viral load, especially when combined with receptor binding domain changes such as D225G can lead to the type of cases seen in Ukraine, where a high percentage of young adults develop infections that destroy both lungsin a matter of a few days.
In healthy individuals, the first encounter with a virus leads to a primary antibody response. When an infection occurs with the same or a similar virus, a rapid antibody response occurs that is called the secondary antibody response. Antibodies are critical for preventing many viral infections, including influenza.
H1N1 1.0 H1N1 1.1
But reinfection may occur if we encounter the same virus before the primary response is complete.
Recently three cases of confirmed infection with 2009 influenza H1N1 were reported in Chile. The first patient had laboratory confirmed infection; treatment with oseltamivir resolved symptoms after 48 hours.
Twenty days later the patient developed a second bout of laboratory confirmed influenza which was treated with amantadine. The second patient acquired laboratory confirmed influenza in hospital, was treated with oseltamivir and recovered.
Two weeks later, while still in hospital, the patient had a new episode of laboratory confirmed influenza infection. Treatment with oseltamivir again resolved the infection. The third patient also acquired laboratory infection in hospital, was successfully treated with oseltamivir, and was discharged. He was readmitted 18 days later with confirmed pandemic H1N1 2009, and again successfully treated with oseltamivir.
These individuals were likely resusceptible to reinfection with the same strain of influenza virus due to a confluence of unusual events. First, all three were reinfected within three weeks, before their primary adaptive response had sufficiently matured. Another contributing factor was the high level of circulation of the pandemic strain. This issue was compounded for patients two and three who probably acquired both infections while in the hospital (called nosocomial transmission).
Could reinfection also occur after immunization with influenza vaccine? Yes, if the immunized individual encounters the virus before the primary antibody response matures, which occurs in 3-4 weeks. This is more likely to occur during pandemic influenza when circulation of the virus is more extensive than in non-pandemic years.
Perez CM, Ferres M, & Labarca JA (2010). Pandemic (H1N1) 2009 Reinfection, Chile. Emerging infectious diseases, 16 (1), 156-7 PMID: 20031070
