HEPA filters cut covid-19 sick days but we’ve been slow proving this

Covid-19 sick days were 20 per cent lower in schools with air-cleaning HEPA filter machines, researchers on an eagerly-awaited study have found. But why has it taken so long to discover how well they work, asks Clare Wilson.

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TEMPERATURES have recently taken a dip in London, where I live, and, at home, I have been digging out my jumpers. At work, the winter feel is reinforced by a surge of interest in covid-19, with rising reported UK cases leading to familiar concerns over pressures on hospitals.

Because there is now much more population immunity to covid-19, it seems unlikely we would go back to restrictions on public mixing and so on. But in hospitals, where people are more vulnerable, there are questions over whether they should be reintroducing measures like staff wearing masks and having stricter isolation policies.

Even if you are tired of thinking about covid-19, the answers to these questions have broader relevance, because the same measures may well also work against other respiratory pathogens, including one that could cause a future pandemic.

Last month, figures showed that during England’s second wave of covid-19, which peaked in January 2021, up to 1 in 50 people in hospitals caught the illness while they were there. This has led to renewed calls from campaigners for stricter measures to stop the spread of airborne viruses by improving air quality.

One hot topic is the use of air-cleaning machines called HEPA filters, which suck in air and absorb tiny particles such as viruses. Some argue that these machines should be put into all hospitals, schools and other public buildings. It might seem like common sense, but we shouldn’t assume they would reduce the spread of covid-19. The machines – each about the size of a tall kitchen waste bin – certainly take viruses and bacteria out of any air that passes through them, but they cannot instantly suck all the pathogens from a room. Whether they have a meaningful effect on cutting infections depends on factors such as the room size and the rate at which people are breathing out microbes.

The only way to know for sure is to do randomised trials where some buildings have the air cleaners and some don’t. Two such trials have been carried out that I know of, both in the UK: one was in Addenbrooke’s Hospital in Cambridge and one in 30 primary schools in the Bradford area. Their results have been eagerly awaited. Now, we have had a sneak preview of the results from the schools. One of the researchers involved revealed at a recent conference that children’s covid-19-related absences were 20 per cent lower in the schools with air cleaners.

It would be nice if it were higher, but I suspect this figure will be welcomed by those who have long campaigned for wider use of the machines. Certainly, any fall in the number of days that children miss school is welcome. We have to keep in mind, though, that this isn’t a 20 per cent fall in their total number of sick days, only the days off caused by covid-19.

On the other hand, if the machines can suck coronaviruses out of the air, they can probably do the same for other viruses, including flu and those that cause coughs and colds, says Mark Mon-Williams at the University of Leeds, UK, one of the researchers.

Ultimately, we will have to wait until the results are published in full to properly evaluate them. And schools will still need to weigh up whether these machines are worth the cost – of several hundred pounds a unit, with two or three needed per classroom – and the fact they are somewhat noisy and can emit a cold draught.

It seems bizarre to me that, even now, three years after these machines started being installed in some hospitals in response to covid-19, we are still waiting to learn how well they work in the form of a published paper. It was a hallmark of the pandemic, though, that while there were plenty of trials of the new medicines and vaccines being developed against the coronavirus, the same scientific approach was rarely taken for physical interventions.

One of the most controversial questions of the pandemic has been over the usefulness of face masks. Wearing a face covering is backed by several major bodies. However, most of the studies cited in support of doing so are “observational”. In other words, they just observed correlations between people’s mask wearing and their infection rates.

Observational studies can be biased by the fact that those who diligently wear masks tend to be health conscious in other ways too, like regularly washing their hands. When respected medical body the Cochrane Collaboration only looked at randomised trials of interventions to promote mask wearing, it found the evidence was inconclusive.

One of the common refrains during the first months of the pandemic was that policy-makers should “follow the science”. But they cannot do that unless researchers have made sure that the science has been done.

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