A week-long trace amount of coronavirus in samples from Fort Smith’s sewage has now disappeared, the NWT government says, but a similar trace has appeared in Fort Liard.
Results of wastewater sampling in seven NWT communities, including Fort Smith and Fort Liard, are routinely made public by the GNWT. The latest results tend to be published roughly every week.
For most of the past week, the territory’s public data suggested there had been a “trace detection” of the virus responsible for Covid-19 in Fort Smith samples taken in mid-March. As of Friday, that warning had been eradicated as samples from the end of March received the all-clear.
A day earlier, the NWT’s chief public health officer had said she was waiting to see if a trend emerged in Fort Smith.
“If the signal amplifies or there’s cause to be concerned that this is a signal that’s increasing, and possibly it’s an unanticipated signal, we would do a public health action,” Dr Kami Kandola told reporters on Thursday.
“We do experience a lot of trace detects. It’s a small signal. It could be transit – someone could have travelled in and travelled out.”
Kandola said only a signal that remained “persistent and amplified” over multiple weeks would give cause for concern.
By email, a spokesperson for the Office of the Chief Public Health Officer added that trace signals meant “a very weak signal has been detected in wastewater samples, often so low that labs are not able to quantify.”
The spokesperson suggested signals from previous Covid-19 infections, already reported weeks ago, could linger at low levels in sewage systems even without Covid-19 being present in members of the community.
“There have previously been false positive signals that take some time for lab analysis to determine,” they wrote by email.
“In the cases of trace signals, we continue to monitor the wastewater in the community to see if a stronger unexpected signal that requires a public health risk assessment is detected.”
While that’s no longer the case in Fort Smith, the latest data published by the GNWT suggests there is now a trace signal of the virus responsible for Covid-19 in Fort Liard’s wastewater.
The same guidance applies. Six out of seven samples collected in Fort Liard late last month had a trace amount of the virus, but the territory is understood to be waiting to see if the signal persists or grows before taking further action.
Sampling of sewage is not a foolproof method of Covid-19 detection as there’s no guarantee a sample taken will necessarily capture the coronavirus even if it’s present more broadly in a community.
Instead, the sampling is designed as an early warning system. If the coronavirus shows up unexpectedly in samples, in levels strong enough to perturb public health officers, acton can be taken.