July 8, 2009

H1N1 “swine flu” virus spreads slowly

While the H1N1 "swine flu" virus circled the globe this spring, causing the World Health Organization (WHO) to declare a level 6 pandemic alert, a team of researchers from MIT and the Centers for Disease Control (CDC) were already investigating why the new virus spread from person to person less effectively than other flu viruses.

"While the virus is able to bind human receptors, it clearly appears to be restricted," Ram Sasisekharan, the Edward Hood Taplin Professor and director of the Harvard-MIT Division of Health Sciences and Technology (HST) and the lead MIT author of the paper is quoted as saying.

Sasisekharan and colleagues compared the new H1N1 strain to several seasonal flu strains, including some milder H1N1 strains, and to the virus that caused the 1918 flu pandemic. They found that while the new strain is able to bind to receptors in the human respiratory tract, the new H1N1 strain binds human receptors much less effectively than other flu viruses that infect humans.

The researchers also found that the new H1N1 strain spreads inefficiently in ferrets, which accurately mimics human influenza disease. When the ferrets were in close contact, they were exposed to enough virus particles that infection spread easily. However, when ferrets were kept apart and the virus could spread only through airborne respiratory droplets, the illness spread much less effectively.

This is consistent with the transmission of this virus seen in humans so far, says Sasisekharan. Most outbreaks have occurred in limited clusters, sometimes within a family or a school.

The danger, says Sasisekharan, is that flu viruses are known to mutate rapidly, so there is cause for concern if H1N1 undergoes mutations that improve its binding affinity. "We need to pay careful attention to the evolution of this virus."

The researchers warn that the new strain might need just a single change or mutation to give rise to strains that are resistant to the influenza drug Tamiflu.

In a separate study, Sasisekharan and colleagues also found that the new H1N1 strain has substantial genetic variability in the proteins targeted by current vaccines, making it likely that existing seasonal vaccines will be ineffective against the new strain.

To date, H1N1 swine flu has been responsible for more than 300 deaths and more than 70,000 people infected, according to the WHO.

SOURCE: Science, July 2, 2009