On Monday, fresh cases once again dropped marginally. Epidemiologists and public health experts say they will have to wait for at least a fortnight before they can declare it as the third wave or dismiss it as just a blip on the graph.
Several factors go into determining the third wave, say experts.
“We will be able to forecast the wave by looking at the graph and working out some numbers for another two weeks. For instance, the rate of transmission of the viral infection or the rate of fall in daily cases will tell us where the graph is heading,” said senior virologist Dr T Jacob John.
There are some worrying factors already. On June 26, the state saw the steepest fall in the average number of fresh cases. The weekly average of case was 7.8% less that day compared to the previous week.
On Sunday, the weekly average of fresh cases was equal to the weekly average the state reported a week ago on July 25.
This meant the rate of decline was zero. On Monday, for the first time since April, despite reporting a marginal increase in cases, the weekly average was higher. It was 1,957 compared to 1,844 seven days ago on July 26.
Epidemiologists say this is the first time the state is seeing an increase in growth rate of the weekly average since April 15.
Another worrisome factor for Tamil Nadu is the increase in R-naught — the average number of people who will contract a contagious disease from one person. The number fell below 1 on May 21 and dropped to 0.6 on June16, according to epidemiologists from NIE.
Now, the R-naught is again more than 1 for Tamil Nadu, they said.
“These patterns look like what we saw in February and March before the start of the second wave. But since numbers are going up and down, we must wait to see if it is going to increase steadily,” said a public health official.
“Until we see a steady increase, we won’t be able to declare it as the third wave,” he added.
Beware of rogue mutation
While these, along with hospitalisations and positivity rate are numbers officials can calculate, any rogue mutation – which is unpredictable – can make things worse.
“A third wave in that case is possible. We will be back to square one at any point of time,” said senior epidemiologist Dr Jayprakash Muliyil, who is also a part of the state task force on Covid.
And even without mutation, with nearly three out of 10 people still susceptible, the third wave stands a good chance, says NIE deputy director Dr Prabhdeep Kaur.
She added: “Get vaccinated, wear masks and do not plan any large gatherings. Delta variant can lead to super spreaders in large gatherings.”