Influenza virus may be transmitted among humans in three ways: by direct contact with infected individuals; by contact with contaminated objects (called fomites, such as toys, doorknobs); and by inhalation of virus-laden aerosols. The contribution of each mode to overall transmission of influenza is not known. But something that most of us touch on a daily basis – paper currency – appears to be able to hold infectious virus for a surprisingly long period of time.
The idea that currency can serve as a vector for transmission of influenza virus is attractive since billions of banknotes change hands daily throughout the globe. To determine if virus can remain infectious on banknotes, a small volume (50 microliters) of a viral suspension was added to a 50 franc Swiss note. The note was kept at room temperature, and at different times the inoculated area was cut out, immersed in buffer, and viral infectivity was determined in cell culture. Infectivity of influenza A (H1N1) and influenza B viruses was detected for only 1 and 2 hours, respectively. In contrast, two different influenza A (H3N2) viruses were detected up to 1 and 3 days.
As expected, the more virus placed on the banknote, the longer infectivity could be detected. Addition of respiratory secretions to the viral inoculum also increased the ‘survival’ time. For example, influenza A/Moscow/10/99 (H3N2) remained infectious on banknotes up to 8 days in the presence of mucus, compared with 2 days without mucus. When higher amounts of virus with mucus were added to banknotes, infectivity could be detected for 17 days. Mucus might provide a protective matrix which slows the loss of viral infectivity.
To determine if similar results would be observed using human specimens, nasopharyngeal secretions from children with influenza-like illness were inoculated onto banknotes. Virus from half of the samples could be detected on the currency for 24 hours, and from 36% of specimens for 48 hours.
These observations demonstrate that influenza virus infectivity remains on banknotes for days. In theory virus could be transferred from currency to the nasal tract by contaminated fingers, initiating an infection. Whether humans acquire influenza by this route is unknown. However, good hand hygiene, which is known to remove influenza virus, is an excellent preventative measure – especially after handling currency.
This study was carried out – where else? – in Switzerland, where 7 million individuals exchange 20 – 100 million banknotes each day.
Thomas Y, Vogel G, Wunderli W, Suter P, Witschi M, Koch D, Tapparel C, & Kaiser L (2008). Survival of influenza virus on banknotes. Applied and environmental microbiology, 74 (10), 3002-7 PMID: 18359825