Sero survey provide information on effect of interventions, trends in new infections

Results of Antigen test being announced at a screening camp in Baiganwadi, Mumbai (Express Photo by Amit Chakravarty)

The word “pandemic” has its origin in a Greek word that means, “all people”. An effective pandemic response, therefore, has to focus on the entire population. The art and science of promoting, protecting and improving health of the entire population is public health. Epidemiology, another word with Greek roots, means the study of what affects the population — an integral part of public health practice. Expertise in epidemiology is especially useful to prepare and respond to disease conditions, when the causative pathogen and characteristics of the population at risk — who, what, where — are not fully understood.

In the COVID-19 pandemic, the pathogen was not well understood, and understanding evolved continuously. Those who have watched the 2011 movie, Contagion, will recall that the characters played by Kate Winslet and Marion Cotillard were the first to be dispatched to respond to the outbreak. They played the characters of epidemiologists. But in real life, experts in public health and epidemiology are not always the first to be tasked to respond.

One tool of epidemiology is the sero-prevalence survey or sero-survey. In the context of the COVID-19 pandemic, sero-surveys have been proposed to test the presence of antibodies against SARS CoV-2 in the general population. These surveys have been conducted in major cities across the world and many states and cities in India. Though there are stated limitations, these surveys provide useful insights on the spread of the infection. Repeat sero-surveys have a few advantages — they provide information on trends in new infections and the effect of interventions on the pandemic response.

Three sero-surveys have been completed in Delhi between June and mid-September. In Delhi’s last sero-survey, the results of which were announced last week, 33 per cent of the samples tested positive. The survey shows 4 per cent additional population — around 6 per cent of the susceptible population — developed antibodies since the last survey period, nearly 30 days ago. From a baseline of nearly 45.6 lakh people with antibodies in the first round, nearly 12.6 lakh and 7.8 lakh new people have been infected in the next two periods of 30 days each. In rounds two and three, for every COVID-19 case officially reported, there are 22 unidentified COVID-19 cases. Most of them are likely to have been asymptomatic. A similar ratio of 1:20 has been noted in sero-surveys in other cities as well.

The repeat sero-survey findings provide important insights on the possible role of herd immunity (HI). Many experts have argued that as the susceptible population goes down and the numbers of those with immunity increase, the pandemic would be over. However, there are two important caveats. One, the detection of antibodies in a sero-survey does not mean immunity. Second, there is no consensus on the appropriate threshold population which should have immunity, threshold herd immunity or THI, before HI comes into action.

From the trends emerging from Delhi’s sero-surveys, even with the very optimistic assumption that everyone who has antibodies is protected, for the hypothetical values of THI of 50 per, 60 per cent or 70 per cent, at 6 per cent of the susceptible population getting infected every passing month, these thresholds can be reached by the end of January 2021, April 2021 and September 2021. This too is very optimistic considering, as we go along, the level of personal protection and public health measures are likely to be enhanced, thus increasing the time needed to reach any THI.

Epidemiology also tells us that the concept of HI is more suitable for large and well-controlled geographical areas and populations. It is not very useful for large cities with wide travel and migrant inflows and outflows. Second, the protection is temporary and location specific. For instance, a person living in Delhi may be protected as a result of HI. However, once out of the city, she or he is at risk. Many experts have also argued that the concept is relevant only in case of diseases against which an effective vaccine is available. In any case, at the national level, it will be long before THI is achieved.

Fortunately, with public health and epidemiology tools at the forefront, we are increasingly developing a better understanding of the pandemic. The components of effective response are a mix of following: All individuals continue to follow non-pharmacological interventions such as face masks, hand-washing and social distancing; governments scale up public health measures of testing, contact tracing, and isolation (whether at home or in facilities) and engage community members; develop a plan for vaccine deployment to make it accessible at the earliest, once it is available; and strengthen but don’t get fixated only on treatment services. These are for only one in every 20 detected cases. Effective public health response can reduce the need for treatment services.

Individually, none of these measures offer a complete solution — they are part of the solution. For example, a pandemic response with far greater attention on hospital beds and ventilators and insufficient focus on public health interventions would be akin to sweeping the floor when keeping the tap running on the full flow. Similarly, even after the vaccine is available, people will need to continue using face masks, washing hands and practising social distancing and resorting to other interventions recommended as per the public health understanding then.

The right combination of these measures can be suggested by experts who rely on continuous evidence generation. That is why the COVID-19 response needs to be guided by experts with skills in epidemiology and public health — that’s why sero-surveys and other public-health oriented tools should be persisted with.

This article first appeared in the print edition on September 23, 2020 under the title ‘Making sense of the virus’. The writer is an epidemiologist and public health specialist. Views are personal

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