Studying Covid-19 data and indicators that assess decline and flare-up in numbers

The number of cases in the three metros that first faced the surge in the second wave of Covid-19 is witnessing a declining trend of daily cases now. This holds true for Delhi, Pune, Mumbai. But is this fall in numbers a real scenario or simply a remnant of fewer tests and missing or incorrect records? The answer determines if the easing of restriction of normal activities is justified. Here is a detailed data-driven analysis to find out the answer.

Are there enough data to come to a resolution?

The data available can be used to ponder on three important questions, if the declining cases actually been a slide down in the epidemic, what are the best indicators to determine so and how effective is the test-trace-treat strategy in a city held as the sample, says the Indian Express.

The data available can help in finding out the test positivity i.e identified positive cases in total tests done and Case Fatality Rate (CFR) i.e deaths to identified positive cases. It is calculated as death on any day over the positives identified 14 days ago or earlier.

How many should be tested

In an ideal test strategy, if a person tests positive, all his contacts in the last few days are tested. But due to constraints like time, money equipment only the ones he was in close contact with (high risk) are tested. Test positivity in such a scenario is high as the likelihood of these people being positive is high. If the identifies positive cases are isolated, there will be low test positivity which means the spread can be contained.

Is test positivity the ideal indicator?

It is good as long as in the earliest stages. But once it spreads sufficiently via a person who was not isolating themselves, the chain of transmission is difficult to trace. . In such a case even when tests are increased the positivity rate might not increase. This means the infection has spread widely with several asymptomatic cases.

Test positivity works as a determinant when the epidemic is declining. When the epidemic nears its end, the tests done decline and positivity too decline or remain the same. However, if the positivity increases even when the number of tests done is less it means there is a need to ramp up testing as the infection chain is still there in the population.

Is test positivity as an indicator enough?

Test Positivity indicator is not enough because it fails to take into account the asymptomatic cases that can infect others. This is when Case Fatality Rate (CFR) and Infection Fatality Rate (IFR) indicators come to play. While the first one determines how many of the identified positives have died, the latter tells how many of the infected persons have died. IFR is much less than CFR as the denominator I.e the unidentified positive cases is much bigger.

By comparing CFR and IFR one can identify the constant for a disease for a given age group. So, even when there is no IFR data, if the CFR is higher, it can be assumed there is a greater number of unidentified infections.

What do the three indicators tell us?

When there is a high positivity rate and high CFR, testing needs to be increased along with a door-to-door survey and contact tracing to identify the spread of the infection before the chain becomes untraceable.

When there is a ‘High positivity rate’ and ‘low CFR’, there is a widespread infection but because of good response by the healthcare system, the fatality rate is low. But this could also imply under-reporting of deaths!

‘Low Test positivity’ and ‘low CFR’ imply the epidemic is under control while ‘low test positivity’ and ‘high CFR’ indicate attempts to intentionally keep test positivity low.

Considering Pune’s epidemic trajectory with mentioned indicators

In the early stages, test positivity was fairly high but cases were low which means there was a large number of unidentified positives and only high-risk contacts were getting tested due to limited tests. During the peak of the first wave, the positivity also shot up indicating the spread of infection quite large and testing capacity not sufficient.

Test positivity remained steady when cases dropped in October until December meaning there was a latent infection in the population. The positivity reached its lowest in January before shooting up again in February 2021 when cases also started increasing, giving forewarning of a second wave,

In March 2021, as testing ramped up, positivity did not reach the levels of the first wave even when the number of cases was much larger. It started dropping before the actual number in April 2020 which implies the health care system was performing better.

CFR on the other hand gives a similar but complementary picture. In May 2020 the high CFR indicates a lack of preparedness for the health care system. After remaining constant for a while, it started rising again in mid-October when it reached its peak as opposed to the peak of cases that was attained in September.

As the CFR remained constant and fairly low since December 2020, it suggests Pune’s health care system performed better. However, a slight increase in CFR in late April 2020 was a forecast of a more infectious second wave and to warn the health system to be more vigilant.

With the above analysis, it becomes clear that transparent data sharing by the administration is most important to understand the trends of an epidemic. Dop in number s in October made the public and administration believe that the worst is over that was not really supported by the indicators such as test positivity and CFR. It. Is important the all-relevant information is available so that the administration can make informed decisions during uncertain times.