Update – Swine Origin Influenza A (H1N1)

2009, Volume 20, Number 2


Danny L. Wiedbrauk, Ph.D.
Scientific Director, Virology and Molecular Biology

On April 15 and April 17, 2009, novel swine-origin influenza A (H1N1) virus (S-OIV) was identified in specimens obtained from two patients in the United States. Since then, S-OIV has spread throughout the world and on June 11, 2009, just 57 days after the virus was first identified, the World Health Organization acknowledged that there was a global pandemic. More than 60,000 confirmed cases have been reported in 100 countries. Reported cases represent just the tip of the pandemic iceberg because not all patients with influenza-like illness are tested and only a fraction of those cases are sent for S-OIV typing and confirmation.

In the continental United States, seasonal influenza A and influenza B activity has declined to undetectable levels but the novel S-OIV strain continues to circulate. This lack of seasonality may prove to be a hallmark of the S-OIV strain and it may frustrate public health attempts to vaccinate the population because the Flu Season may extend throughout the year. It is also likely that influenza testing will be done during the entire year.

S-OIV and Rapid Influenza Tests. For detection of seasonal influenza, the sensitivity of rapid influenza tests is approximately 50-70% compared with viral culture or PCR. The CDC reports that the sensitivity and specificity of rapid tests is not yet known for S-OIV and the CDC has received anecdotal reports of false positive and false negative results. Thus S-OIV infection cannot be excluded when a patient tests negative for influenza A by rapid antigen test, particularly at the beginning or end of the influenza season. Rapid antigen tests cannot distinguish S-OIV strains from seasonal influenza A strains.

Influenza Typing. Warde Medical Laboratory is working with the Michigan Department of Community Health to validate the S-OIV detection and confirmation protocols developed by the U.S. Centers for Disease Control and Prevention. Once these protocols have been vetted, Warde will be able to differentiate S-OIV strains from seasonal influenza A strains. While this level of differentiation is not necessary for all influenza-like illnesses, it is important for patients who receive antiviral therapies. Seasonal influenza A strains are resistant to Tamiflu (Ostelamivir). S-OIV strains are resistant to amantadine and rimantadine (Table 1).

Table 1. 2009 Antiviral susceptibility information for seasonal influenza A strains and the novel swine-origin influenza A (S-OIV) H1N1 strains.

Antiviral DrugSeasonal Influenza AS-OIV H1N1 Strains
Ostelamivir (TamiFlu)ResistantSusceptible*
Zanamivir (Relenza)SensitiveSusceptible

Source: Centers for Disease Control and Prevention.
* There have been recent reports of Tamiflu resistance in Australia