Schools may have had more Covid-19 infections before Christmas than previous research showed, according to data from a pilot study that senior public health experts believe could provide a crucial early warning system against future outbreaks.
Covid was present in the sewage of 80% of 16 primary and secondary schools in England during December, and researchers detected the virus about a week before community testing, according to Mariachiara Di Cesare, a senior lecturer in public health at Middlesex University who led the study.
“In the earlier weeks, it was around 20% [of schools],” she said. “Starting from the end of November, we saw an increase in the percentage that returned positive. And that was in line with a lag of a week before what was happening in the community.”
Di Cesare and other researchers from Term, a collaboration involving universities and the Joint Biosecurity Centre, saw a steady increase to 50% then 80% of schools taking part in the project, at a time when cases nationally seemed to be falling. “We were really worried, but the samples were consistent in the school and in other schools in the area, so we could see the virus was circulating,” she said.
Di Cesare was keen to emphasise that the study, which took samples every five minutes from each school’s wastepipe for eight hours a day, had been attempting to show that wastewater could be used to discover Covid outbreaks, rather than their scale or how transmission might be occurring.
“This is not data we would expect to be used to close a school,” she said. “One of the problems we are working on is how to communicate the data to public health teams.”
The small sample size also means the study may not be representative of schools across England. A study by the Office for National Statistics of 105 schools in November, using PCR testing, found the virus in 55% of schools in one day of testing, although it also said the data could not be used to extrapolate to the whole of England.
However, because wastewater seems to provide earlier signs of Covid infection, public health officials believe it is better and cheaper than relying on PCR or lateral flow testing.
Maggie Rae, the president of the Faculty of Public Health, said having an effective early warning system in place was vital for preventing further outbreaks. “Test and trace is not cost effective and it’s not an early warning system,” Rae said. “Testing doesn’t tell us how many people have got the virus, just those who have come forward for a test,” she added.
“Wastewater could give you a very very good sense of unknown infections that you can then track.”
Local authority public health teams could potentially identify an area where Covid may be about to break out, and go door-to-door offering advice, support and testing equipment, she said.
The water supply and sewerage systems have been used for public health ever since John Snow, a London physician, proved in 1854 that cholera was spreading from a well in Soho.
The National Wastewater Epidemiology Surveillance Programme began looking last summer at whether Covid could be reliably detected in sewage, taking samples at 96 treatment plants in England, Wales and Scotland.
Andrew Singer, who leads N-WESP, said they were able to detect Covid in the sewer system when at least one in 10,000 people is infected – about 15 people in a city the size of Oxford.
The growth of new Covid variants is also detectable, he said. “You can get an early insight into variants of interest and quickly assess how worried we should be about them.”
The programme is being expanded to more than 200 sites in England, covering 80% of the population. Sewage is routinely monitored in Australia to detect Covid outbreaks, and before the pandemic wastewater analysis was part of polio and norovirus detection in the UK.
“We have the oldest sewer network in the world, practically. When you compare that with Australia’s, which has pipes that are 20 years old, and documentation that shows where they are – to be honest, I’m envious how easy it is.”