Last updated January 22, 2018 at 1:40 pm
There’s no escape – sneezing and coughing isn’t needed to spread influenza.
Australia might be in the midst of a heatwave, but the “Australian Flu” is going strong in Europe. The H3N2 strain of flu that swept through Australia during our winter has hit Europe in a big way, with deaths in the UK triple that of last year.
But one of the big assumptions about how the flu spreads from person to person has just been busted by new research. Sneezing and coughing probably isn’t responsible for spreading infectious viruses, it’s all the other times when someone is breathing normally that you need to be worried about.
How viruses are transmitted
Exactly how influenza and other infectious viruses are transmitted has been unclear due to the lack of human data on the amount and infectiousness of influenza virus in exhaled breath. Despite this, it was assumed that sneezes and coughs were the major mechanism for spreading the virus into the air.
To clarify exactly how much, and when, the virus is shed from the body, US researchers collected the exhaled breath from over 140 people confirmed to have influenza. For each of the first three days of symptoms, the patients collected 30-minute samples of exhaled breath, and spontaneous coughing and sneezing.
The researchers detected infectious virus in 39% of the samples, demonstrating that a significant fraction of influenza cases routinely shed infectious virus into aerosol particles small enough to remain suspended in air. These are a significant risk for airborne transmission to uninfected people nearby.
What did surprise the researchers however was that 48% of the fine aerosol samples that didn’t involve coughs also had detectable viral RNA. In other words – half of the time, normal breaths contained viral RNA that could go on to infect others. While coughs did cause virus to be expelled, this constant exhalation of infection particles means they’re not required for transmission of the virus from one person to another.
Additionally, it’s not as if sneezes were a massive concentrated dose of virus all at once. The sneezes that were collected and analysed did not have more viral RNA in them than the normal breaths or coughs. Given their relative rareness, the researchers suggest amount of virus expelled over time wouldn’t be a significant part of virus shedding. However, a sneeze could potentially spread the virus load further than normal breathing (the experiment didn’t look at range, only quantity), and they can cause the infection of surfaces.
Sneezing and coughing not needed
According to the researchers, the results show that sneezing and coughing were not necessary to create aerosols containing the virus. Indeed, they pointed out that sneezing is actually relatively rare, or at least was in their samples, and therefore is probably not an important factor for the spread of influenza.
This revelation could change the mathematical models of infection spread, and the messages that are given to us on how to limit our exposure (and exposure of others).
The researchers also showed that the amount of virus being exhaled depended on whether the infection was in the upper or lower respiratory tract. If the infection was in the lung, the virus was more likely to be exhaled.
While we might all shrink away from someone who is coughing and spluttering on the bus in fear of catching their infection, it’s actually their normal breath that has probably put us at risk. No longer can we rely on obvious clues of whether or not someone is likely to make us sick, it could be happening to us by stealth.
At the end of the day this is just further proof that the best thing to do if you’re sick is just to stay home, away from others.
The research was published in the Proceedings of the National Academy of Sciences