The BA.4 and BA.5 variants of Omicron have conquered Dunedin.
Viruses such as Covid-19 mutate over time, and following the initial surge of Covid cases in New Zealand, the increasingly more transmissible Delta and then Omicron variants have afflicted the country.
The largest outbreak of cases experienced so far was the initial Omicron wave, but the current explosion of cases across New Zealand has been driven by the emergence of several subvariants of Omicron, mainly BA.2, BA.4 and BA.5.
In early July BA.4 and .5 overtook the BA.2 variant of Omicron to become the dominant strain, and BA.4 and BA.5 have since become the cause of more than 90% of infections.
However, in a few areas — including Dunedin — rates of BA.2 remained high.
Until last week, wastewater tests at Dunedin’s Tahuna testing point recorded an 18% rate of BA.2, well above the national average: in contrast, other southern testing sites at Mosgiel and Queenstown were showing little or no traces of BA.2.
That changed this week, with BA.2 losing its battle against the stronger variants and dropped to a 0% reading.
"We are working to understand the sensitivity of this, for example, how many cases you need in a catchment of a variant," Institute of Environmental Science and Research health and environment senior science leader Brent Gilpin said.
"Uncertainty around these could easily be +/-10% or more, but the key thing is the trends over time and average, which is now greater than 90% BA.4/BA.5, and which is pretty consistent across sites."
ESR would continue to monitor what variants of Covid-19 were showing up in wastewater as unusual spikes of a variant could reflect new cluster of cases infected at the same time, Dr Gilpin said.
There were 4818 new cases of Covid-19 reported by the Ministry of Health yesterday, 325 of which were in Otago and Southland.
The deaths of a further 23 people who had Covid-19 were reported, one of which was in the South.
