Not all flu viruses are the same. Some are hardy survivors.

Some influenza viruses can survive and remain infectious on the fingertips for at least 30 minutes, long enough to transmit the flu to oneself or others, says a study in the January issue of Clinical Microbiology and Infection.

Flu is transmitted from person to person mainly by inhaling tiny virus particles that are released into the air when infected individuals cough and sneeze. Direct contact with contaminated fingers is considered an equally important method of transmission. Researchers sought to find out if the type or size of the flu virus affects its survival.

Experiments in Geneva involved two common influenza viruses: H3N2 and H1N1, the virus that triggered a global flu pandemic in 2009. Both viruses are included in the 2013-14 flu vaccine and are causing most of the world's current flu illnesses, according to a recent World Health Organization bulletin.

Six volunteers with experience handling laboratory viruses were recruited. In separate experiments, droplets of H3N2 and H1N1 mixed with respiratory secretions from infected patients were deposited on three fingertips of each volunteer. The subjects' fingers were undisturbed and kept at room temperature before testing for the presence of viral particles at various intervals. Particles were classed as "survived" if they were capable of propagating in laboratory cell cultures.

Both flu viruses were infectious on all 18 fingers after one minute. At three minutes, H3N2 was infectious on six fingers while H1N1 remained infectious on 15. At five minutes, H3N2 and H1N1 were infectious on five and eight fingers, respectively. H1N1 remained infectious on five fingers after 15 minutes compared with four fingers contaminated with the H3N2 virus. After 30 minutes, the number of infectious fingers dropped to two for each virus.

Larger droplets of H3N2 and H1N1 were found to contain a higher concentration of viral particles and remained infectious longer than smaller droplets with fewer particles. But smearing flu droplets over the fingertip reduced their infectiousness compared with untouched droplets, the study found.

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