Chandre Dharmawardana, whose preferred choice of title is“Is complacency marring Sri Lanka’s Covid-19 effort? Fatality figures and the number of “confirmed cases”
Professor Pranna Cooray, in a Q&A session (Island, 20-04-2020) draws attention to Sri Lanka’s Covid-19 Case Fatality ratio (CFR). This is the ratio of the number of deaths to the number of confirmed cases. He pointed out, using the April 13th data for the CFR figures for neighbouring countries, viz., Sri Lanka’s 3.2 percent (total cases – 218), India 3.4 percent (10,541), Pakistan 1.7 percent (5,716), Bangladesh 4.9 percent (803), “that it is unacceptably high” given Sri Lanka’s reputed public health system.
In the figure, Sri lanka’s offical data from up to 14-04-20 are analysed into two groups, viz., “confirmed public cases”, and total confirmed cases, where the latter includes cases inside quarantine centers, and hence NOT in the public pool. The data are modeled using a mathematical model similar to that for the competition between species in an ecosystem. The change in trends after imposition of the curfew shows that such methods are effective. The predictions for the two sets of data hold from the 14th up to about the 21st (i.e., for about a week). Then a new trend is seen.
Professor Cooray argues that “our inability to find the true infected load, or even to come near that, is reflected in our Case Fatality Rate (CFR, percentage of the number of deaths divided by the total number of positives)”. So he advocates doing more tests to determine the “true infected number”. Other health experts have also called for more comprehensive testing.
Until results form such testing comes, we can turn the argument around, and ESTIMATE a likely number of “confirmed cases” if we knew what the CFR for Covid-19 for a tropical country should be. In a report published in the Lancet only weeks before the interview with Prof. Cooray, the famed Harvard epidemiologist Marc Lipstich is seen debating other expert authors on what the CFR should be.
Prof. Lipsitch writes “No expert thinks the 3·6% raw ratio of deaths to cases on March 1 is an accurate estimate of the CFR because it suffers from all these … biases. The authors make the situation worse … without correcting for ascertainment of mild cases inflating the estimates, bringing them further from what most experts believe are the true numbers, around the 1–2% range for symptomatic cases”.
I have also assumed that a proper value for the CFR in Sri Lanka might be about 1.8% in my models of Covid-19 (See Figure). The number of deaths so far reported in Sri Lanka is seven – too small to be confidently used in statistical analysis. However, if we use this fatality number, the “confirmed cases” should be 350-450 if full testing had been feasible. However, the reported confirmed cases stood at 218. So, the further discovery of 15 cases (all in quarantine centers) on 14th of April, then 5 new cases of which 4 were in quarantine centers on the 15th, and so on to the discovery of 33 new cases on the 20th of April, 38 new cases on 23rd April etc., WITHOUT the death toll increasing should not be regarded as too worrisome. But there are other troubling signs.
So, assuming as Prof. Cooray does, that all the infected cases have not been discovered, one can expect that the number of confirmed cases will indeed increase towards 350 to 450. But the graphs suggests that the upper bound of ~450 may not be valid! Is the fatality number 7 valid?
THIS SHOULD BE REGARDED AS A CLARION CALL TO CREATE ADEQUATE HOSPITAL CAPACITY AND FURTHER TESTING.
The early successes in the fight against Covid-19 has led to a complacency and a belief that putting in curfews and imposing physical distancing have solved the problem. However, the graph shows a definite new trend since 21st April due to hidden public cases. They should have been in quarantine.
The case of the Piliyandala fish vendor in instructive. An infected arrival was presumably “quarantined” in the same house as the vendor? Given the housing, and the social habits of the populace, “agreeing to quarantining at home” is perhaps like the promises of politicians. Instead, all suspected cases must go to monitored quarantine and anything less is a waste of the expense and effort implied in having curfews and lock-downs. Releasing after 14 days should be replaced by the principle of “releasing only if tested negative”.
Although new arrivals now face mandatory quarantine, people sent off to “self-quarantine” since early-March probably lived with others and are now a reservoir of infectious people who should have been tested. Adequate arguments for “herd immunity” from such a reservoir do not still exist. EVERY CASE of respiratory illness referred to medical officers should be tested for Covid-19.
According to the Institute Pasteur, some 17% of infected people show no symptoms. So, using our estimate of some 400 infected people, there should have been some 70 more asymptotic carriers in the community by 14th April. The challenge is to prevent its increase without becoming complacent.
If the incubation period is 14 days, curfews and physical distancing into early May should be effective in low-incidence areas. However, the Western Province (WP) is the hub of economic activity. It contributes 42% to the GDP, and 60% to the service sector. Opening the country with the WP closed is like expecting a one-legged, one-handed, partially traumatized person to take his usual leadership role of the country. So it is in regions like the WP, that strict quarantine and more testing are needed.
However, even if Sri Lanka had the means of a Germany, comprehensive testing is impossible within the required time. Germany carried out some 350,000 tests PER WEEK and managed to examine only about a million people, in a country of 67 million. That is, only 5 tests per 1000 people. South Korea did not do much better.
We have not been able to find out how many tests have been carried in Sri Lanka since 10th March, although one suspects that the number is about 6000-8000 for a population of 22 million. Kerala with 36 million people has tested some 20,000, compared to Canada’s 460,000 with a comparable population. However, it is not the number of tests per 1000 that matters, but the STRATEGIC deployment of the available testing.
Testing a person once is not sufficient due to false negatives and false positives in tests. Even with the resources of a Germany, to test ALL the people in Sri Lanka’s western province may take four months, while to adequately test (i.e., 1/e of them, e = 2.718, base of Napier’s Logarithm) will take one and a half months. But we need the information NOW to prepare the hospitals for a possible surge. This is why estimates from small amounts of strategically collected data using models (however limited they be), together with strategic inputs from hospitals may become relevant. However, epidemiological models (or weather prediction models) fail beyond even a week. Simpler indicators, e.g, the constancy of the fatality figure for a few weeks, would be a better harbinger of the end of the crisis. Even then, strategic testing must continue.
The method of tracking social news, i.e., gossip and information, spot checks, raids, and all the methods employed for the detection of criminals and terrorists become equally effective in tracing absconding sick people and their asymptomatic contacts who may be infectious. So, optimal selection of samples of people from areas under suspicion coupled with an investigative approach is the best strategy. I have discussed this already in my article in the Island, 6th of April (http://www.island.lk/index.php?page_cat=article-details&page=article-details&code_title=220926).