A study that involves infecting humans with the H1N1 influenza virus has uncovered information about the minimum dose of virus needed to produce mild-to-moderate illness, scientists in the US have announced.

The study, undertaken by the National Institutes of Health (NIH), also revealed how much time elapses between a known time of infection, the start of viral shedding, which is a sign of contagiousness, the development of an immune response, and the onset and duration of influenza symptoms.

It is hoped the findings from the study can help provide a foundation for more rapid, cost-effective clinical trials to evaluate new influenza drugs or to determine the efficacy of a candidate vaccines for both seasonal and pandemic influenza.

H1N1 influenza virus, also known as swine flu, was identified in 2009 when it quickly spread to a global pandemic. By the start of 2010 it had caused 17,000 deaths.

“It is hoped the findings from the study can help provide a foundation for more rapid, cost-effective clinical trials to evaluate new influenza drugs.”

The symptoms of the flu are the same as the seasonal flu, and treatments include antiviral drugs such as Tamiflu and Relenza.

However, it was identified as being different from previous swine flu strains because it combined swine, bird, and human flu viruses.

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The NIH study was conducted under controlled conditions on 46 volunteers who were divided into five groups and exposed to influenza virus in escalating doses.

The virus, though synthesised in a lab, was genetically identical to the virus that caused the 2009 H1N1 pandemic influenza.

Once exposure was complete the volunteers were closely monitored in isolation.

When the scientists administered an influenza virus dose of 107 TCID50, a measure of the amount of virus required to produce cell death in 50% of cultured cells inoculated with virus to 13 of the volunteers, 16% shed the virus and developed symptoms.

The scientists discovered that lower dosages did not generate responses that met this threshold, leading them to establish the minimum dose of influenza virus needed to produce mild-to-moderate illness.

Researchers from NIH’s National Institute of Allergy and Infectious Diseases (NIAID) presented the preliminary study results at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) meeting in Denver earlier in September 2013.