Why dumping TPR for WIPR won't help Kerala beat COVID

After much hue and cry over test positivity rate (TPR), the imprecise metric for determining imposition of lockdown in an area for the purpose of COVID-19 mitigation, the government of Kerala has now come up with a new metric. It is called weekly infection population ratio (WIPR).

WIPR is the total number of COVID-19 infections reported in a week adjusted to the population of a locality. If the WIPR is above a threshold, the government claims stringent lockdown measures will be instituted.

The new metric misses the mark, yet again. First, let us set the rules of this game of Tom and Jerry (mouse being SARS-CoV2 and the cat, our government). Cat Tom wants to catch up with mouse Jerry. To do that, Tom needs to first understand Jerry well. Let’s try to understand the mouse, or, in other words, the virus.

The majority of SARS-CoV2 infections are asymptomatic. That is one reason why the serosurveillance study showed high rates of antibody positivity, which is several folds the actual numbers of cases identified. A majority of symptomatic SARS-CoV2 infections, COVID-19 disease, resolve without causing any serious issues for the patients. Only a small proportion of patients develop serious disease, requiring the use of oxygen or intensive care therapy. The elderly and those persons with health conditions such as diabetes and obesity are more likely to develop the severe disease.

Vaccination is the only way to protect an individual from the virus. But, what is the impact of vaccination against the delta variant, the current variant of the virus that is causing the majority of the infections? The effectiveness of most vaccines against breakthrough delta variant SARS-CoV2 infections is very low after only one dose (around 30 per cent). The effectiveness of vaccines rises to around 80 per cent after two doses. Most importantly, the effectiveness of vaccines against hospitalisation from delta variant SARS-CoV2 infection after one dose is around 80 per cent and after two doses is around 96 per cent.

In simple words, vaccination is able to protect an individual from developing severe disease even after one dose, but may not prevent the infection in itself.

SARS-CoV2 infections will create havoc in our healthcare system if it leads to overwhelming of the healthcare resources. By now, we have built capacity. Besides, 80 per cent of residents in Kerala over the age of 45 have received at least one dose and 42 per cent have got two doses. In that case, the chance of infections in vaccinated individuals remains significant, but the chance of the severe disease requiring hospitalisations remains trivial. The rate of hospitalisation must be the only crucial factor in this Tom and Jerry game. And, the only way Tom (government) can really catch up with Jerry (SARS-CoV2) is by getting some extra power in his strides (vaccination).

Here is the problem with the WIPR. It only looks at the number of infections in a locality. It does not take into account the likelihood of those infected individuals having received vaccination against the infection. Two localities in Kerala can have the same number of infections, or WIPR, but have varying degree of severity of disease.

WIPR misses the mark where it matters. There must be a way to adjust for the rates of vaccination in a locality with the WIPR to obtain meaningful trends. More importantly, such a metric must also reflect a metric that gauges the healthcare resource capacity, such as the number of persons who require oxygen and intensive care therapy.

Until we use a scientifically rational metric, sadly, the cat will continue running after the mouse.

The author is an oncologist and epidemiologist.

The opinions expressed in this article are those of the author's and do not purport to reflect the opinions or views of THE WEEK.