Bengal’s ‘Sentinel Survey’ shows encouraging results, but….
In the middle of last month, with the 2nd wave showing signs of tapering off, the Health Ministry of the West Bengal govt. initiated a ‘sentinel survey’ to probe the present status of COVID-19 in the state (pic 1). The survey would track the movement of the new virus (well, not so new anymore) and the disease it causes in all 23 of the state’s districts. It was hoped that this would help the authorities be better informed and ring the alarm bells whenever the disease was on the verge of flaring up in any region.
The sentinel survey will be carried out in phases over the next few months. However, the initial results have been published in prominent newspapers and is worth looking into. Here is a short summary.
Result 1: Vaccines provide significant protection
The sentinel survey provides what is perhaps the first round of ‘official evidence’ that those who have between vaccinated are significantly protected against the disease (similar conclusions have been published by couple of media reports, including the author). The headline of the well-known Bengali daily Bartaman, which has a decent science-section, states (pic 2) that the vaccines provide a protective effect of 86%. The news report elaborates that ‘out of 4276 individuals who responded to the survey’s queries about vaccination, 623 had been vaccinated while 3653 had not received their shots. 43 of the unvaccinated individuals had come down with COVID while only one of the 623 vaccinated had fallen sick. The comparative analysis shows that the vaccines provide a protective effect of 86%’.
That is undoubtedly encouraging. Of course it is not really novel; it is in pleasant agreement with the several analyses published in reputed journals and media reports over the last 6–7 months. While infection may still occur (it is an airborne virus after all), there is plenty of data from several countries that vaccination does provide good protection against severe disease, ICU and deaths. And, the latest of results show this holds true even against the Delta variant. Vaccinate more, vaccinate well, vaccinate fast if you want to move ahead of this pandemic.
I have not been able to read the survey-report itself. Its unlikely to be in the public domain. And the newspaper reports do not talk of any difference in protection between people who received only the first dose versus those who have been completely vaccinated. That’d be an interesting piece of data not for only academic interest, but for the govt’s policy makers, especially cos recent studies have shown that even the first vaccine shot provides >60% protection against the disease.
In this context, it’d be appropriate to inform the reader that of the 72 million+ citizens of West Bengal who are 18+yrs old (and hence eligible for vaccination) 12.2% have received both doses while ~18% have received the first vaccination shot. That means, ~30% of the state’s adult population have at least some degree of vaccine-induced protection against COVID as of now. This is a decent figure among India’s big-population states. Of course, there are regional discrepancies. Kolkata (or as I still love to say ‘Calcutta’) and the neighbouring district of North 24 Paraganas have consistently been the epicentre of the pandemic in the state and so it is not surprising that a substantial part of the vaccine-drive has focused here. As of 2nd Aug, ~55% of the citizens of Kolkata+North 24 Paraganas have received at least one shot of the vaccines, and that is rather reassuring. But it is also evident that the vaccination-drive in the rural areas needs to pick up. Sure, India’s lackadaisical vaccine procurement has bogged down the rate of vaccination, but providing at least the first shot to people in villages in the state might be useful in buffering the thrust of the 3rd wave, whenever it arrives.
Result 2: How many people in the state are infected but asymptomatic?
Finding out what % of the state’s population are COVID-positive but asymptomatic (or at least almost-asymptomatic) has been a major objective of the sentinel survey. The importance of this is self-evident. Globally, it is well-known that those who are infected but do not know that they are (cos they display no symptoms, even to themselves) are the major drivers of the pandemic. How many such people in West Bengal? In which areas? Tracking them is central to stopping the transmission of the virus in society.
To provide itself with these answers, the survey had shortlisted 28 (big, mainly public) hospitals with at least one from each of the 23 districts. Each hospital would collect ~400 samples from ‘non-COVID ward/general’ patients i.e. patients of the Opthalmology, Orthopedic and Gynecology and Obstetrics wards of these hospitals. Some samples were also collected from people who had visited the Outdoor wards for consultation. This resulted in a total of 10000+ samples and they were tested for presence of the SARS-CoV-2 virus.
The results can be considered optimistic (pic 4) cos, on average, only 1.1% of the ‘non-COVID’ people tested show presence of the pandemic-pathogen. Of course, some districts — Darjeeling, Nadia, both Midnapores, Alipurduar — show a higher percentage of asymptomatic patients from these hospital depts. But overall, it seems to be a better-than-expected situation.
I wrote seems to be cos, this result troubles me. Isnt there a sampling bias here?
To elaborate, do note that this ‘only 1% asymptomatic’ has come from samples taken from 3 hospital depts — Opthalmology, Orthopedic and Gynecology and Obstetrics. Question is — are people admitted to these depts a good representative of the various socioeconomic tiers of the state’s population? Will sampling them give a holistic picture of the state’s asymptomatic population?
I do not think so. The survey certainly has some relevant data, but it is not enough. This is because people admitted to these hospital wards are there precisely because they are suffering from some ailment of the eyes (maybe cataract), or bones (maybe fractures). Think about it — these are things that make people more cautious and impede their travel. They are at various stages of rest and recuperation. Their disease has made them less mobile than many others in society.
The same holds for samples taken from women in the Gynecology and Obstetrics wards. A significant number of them must be pregnant, and it is no brainer that pregnant women are usually quite careful. Moreover, a year and half of pandemic has certainly heightened that caution by the women themselves and all who care for them. Aren’t such individuals less likely to be exposed to the virus compared to the more active and outgoing working population?
Although the newsreports have no information on it, another ‘bias’ factor could be at play here — age. Again, think about it. People of all age groups do not visit these hospital depts for check up. A public hospital’s Opthalmology dept, for eg, is likely to have midaged and elderly patients. Similarly, the Obstetrics dept is unlikely to have women of all ages in its list.
Taken together, it’d not be incorrect to say that there is some sampling bias in the present form of the survey. Of course, it has data — it shows the status of patients who are at these hospital depts. But this study must be supplemented with further surveys. Especially, since COVID spreads largely through the ‘working indoor populations’ and younger, apparently-healthy ‘working age’ people are more likely to be asymptomatic, there is an urgent need to test those who work in AC offices, banks,malls and shops, traditional shaded bazaars, those who have visited religious and social gatherings and daily commuters of crowded buses, local trains and AC cabs. Only then will we build up a complete picture of the silent spread of COVID in the state. Else, such results might result in a ‘false negative complacency’ while the danger lurks close by [ do note that a major reason why Kerala’s numbers are high cos their testing and tracing is working in an epidemiologically-relevant way….]
Didn’t the surveyors notice this problem in their analysis? I cannot speak for them. It is likely they did but were rather helpless about it. This is because a newsreport clearly reads that ‘house-to-house surveys have been put on hold due to lack of adequate infrastructure and funds. Hence the present focus is on surveying the hospital depts’. The problem described is a non-trivial one, and I’d like to believe that the surveyors went ahead with their efforts hoping that a partial understanding is better than doing nothing; twilight is better than sitting in the dark.
Thats the lot for the day. But, before I conclude, a glimmer of hope. Have you noticed the recent 4th national serosurvey says that >50% of young children showed presence for anti-SARS-CoV-2 antibodies (pic 5)? Well, thatz striking cos none of them have been vaccinated. Then? Well, there is only one other way — natural infection. The results indicate that a large part of Indian children did get infected with the virus.
No, it is not alarming. Rather, it is a bit reassuring; cos although such a large number of children seem to have been infected (and fought off the infection — the antibodies prove that) , isnt the number of children who actually came down with a severe form of the disease or died, really really less?
More on this later. Some serious reading needed. But as the experts are already saying, not much scientific reason to believe that children will be badly hit by the 3rd wave. Rather, Nature seems to have made human kids stronger than adults at least in this aspect.
REFERENCES
https://bartamanpatrika.com/home?cid=13&id=308907
