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. 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
In “Influenza virus attachment to cells” we left the intact virion on the cell surface. The next step is that the viral genetic information – for influenza virus, the individual RNA segments – must enter the cell so that it can be reproduced. The mechanism for influenza virus, illustrated below, involves a step that is a target of the antiviral adamantanes.
The entry of influenza virus into cells is one of the best undertood of all known viral entry mechanisms. After the virion attaches to sialic-acid containing receptors at the cell surface, the virus-receptor complex is taken into cells by endocytosis, a process by which cells normally take up molecules from the extracellular fluid. As the endosomal vesicles that contain the virus particles move towards the cell nucleus, their pH drops. This change in pH is accomplished by a cellular channel that pumps protons (H+) into the vesicle. When the endosomal pH reaches 5.0, the viral HA protein undergoes a conformational rearrangement. This change exposes a fusion peptide on the HA – a short, hydrophobic sequence that inserts into the endosomal membrane and causes it to fuse with the viral envelope. When this occurs, the viral RNAs are released into the cytoplasm. They are then transported into the cell nucleus where viral RNA replication occurs.
In the influenza virion, the viral RNAs are not naked, but bound to a number of viral proteins, including the M1 protein. This protein forms the shell that underlies the lipid membrane of the virion. Unfortunately, if the viral RNAs are bound to M1 protein when they are released from the virion, they cannot enter the nucleus. To get around this problem, the influenza virion has a few copies in its membrane of a protein called M2. This viral protein forms a channel in the membrane that actively pumps protons from the endosome into the interior of the virion. These protons lower the pH in the interior of the virion, releasing the viral RNAs from M1. In this way the RNAs can enter the nucleus.
The M2 ion channel is the target of the antiviral adamantanes, depicted below. These compounds clog the channel and prevent it from pumping protons into the virion. In the presence of adamantanes, viral RNAs remain bound to M1 and cannot enter the nucleus. Therefore viral replication is inhibited. Resistance to adamantanes occurs by changes in amino acids that line the M2 channel. These changes prevent the drug from plugging the channel.
The CDC has issued a health advisory which indicates that a high number of currently circulating H1N1 influenza viruses are resistant to the antiviral drug oseltamivir. Fifty H1N1 isolates obtained in the U.S. since October 2008 were examined for drug sensitivity; 98% of these isolates are resistant to oseltamivir. Influenza H3N2 isolates, however, are still susceptible to the drug. The H1N1 isolates remain sensitive to zanamivir and amantadine. As a result of this finding, the CDC suggests that for treatment of influenza, “zanamivir or a combination of oseltamivir and rimantadine are more appropriate options than oseltamivir alone.”
Oseltamivir and zanamivir are recently developed influenza antiviral drugs that inhibit the viral neuraminidase. This enzyme is required for infection and spread, and consequently its inhibition reduces viral production and disease. Both inhibitors were designed to mimic the natural ligand of neuraminidase, sialic acid. In theory, the more an inhibitor resembles the natural compound, the less likely the target can change to avoid drug binding while maintaining viable function. The results with oseltamivir do not support this theory. During the 2007-08 influenza season, 10.9% of H1N1 viruses tested in the U.S. were resistant to the drug. Although it is not known what percentage of the 2008-09 isolates will be resistant to oseltamivir, it is clear that resistance to this drug is rising.
It is not surprising to observe increasing resistance to oseltamivir, given the high mutation rates of RNA viruses. What is somewhat surprising is the absence of resistance to zanamivir. This difference may be in part due to structural differences between the two compounds – perhaps it is more difficult for the neuraminidase to be resistant to zanamivir and still have enzymatic function. The ways that the drugs are administered may also play a role – oseltamivir is taken orally, a route that is more widely accepted, while zanamivir is given by inhaler. Consequently, it is probably used less than oseltamivir.
One consequence is probably certain: the CDC recommendation that the two neuraminidase inhibitors be used in combination this year is likely to increase resistance to both neuraminidase inhibitors. This will be a difficult problem because there is widespread resistance among H3N2 strains against the only other licensed anti-influenza virus drug, the amantadines.