How much has COVID-19 spread in India? What recent serosurveys tell us

Results from Ahmedabad, Chennai, Pune and Delhi

virus-cells-coronavirus-copy-space-virus-Covid-19-shut Representational image

serosurvey conducted in the Greater Chennai Corporation areas in August has revealed that one-fifth of the population has been exposed to COVID-19. The study, interestingly, showed a huge variation in sero prevalence across regions, ranging between 7.1 per cent in Madhavaram and 44.2 per cent in Tondiarpet. A serosurvey involves testing of blood serum of a group of individuals for the presence of antibodies against that infection to know who has been infected in the past and has now recovered.

The sample size was around 12,000. Of the individuals tested, 2,673 individuals had evidence of past SARS-CoV-2. "The reading [inference] is 21.5 per cent of Chennai's population reached the immunity levels by July-end," GCC commissioner G. Prakash told reporters. "Now, the [immunity] levels will be more and it would also mean a [comparatively] lower transmission rate," he said adding the scientific study would be useful to alter strategies to tackle COVID-19.

"Around 80 per cent of the population is still susceptible to SARS-CoV-2 and we need to understand the reasons for varying seroprevalence across zones to improve our control measures," the civic body said.

Serosurvey tests done across the country are giving us a good indication of the actual number of infected cases in the country, which are exponentially larger than the cases actually reported or tested. The ICMR has announced that it will conduct a national serosurvey in the coming months. The previous such survey conducted in May had revealed that 0.73 per cent of the population surveyed showed evidence of past exposure to SARS-CoV-2, and that urban slum population was the most vulnerable to the infection. This came at a time when the official COVID-19 tally in India had just breached the 3 lakh mark.

So, what do some of the most recent surveys tell us? 

How are serosurveys generally conducted?

A serosurvey involves testing of blood serum of a group of individuals to determine the presence of antibodies against that infection. People from Red, Orange and Green zones in a district will be tested for COVID-19 at random to check if they have developed antibodies against the infection even though they are asymptomatic or show mild symptoms. The survey will involve collection of venous blood samples from 400 randomly selected individuals (one per household) from 10 clusters in each selected district, divided into low risk and high risk groups. 

Throat and nasal swabs will be collected for RT-PCR tests and samples should be tested in a one-time pool of 25. "In addition to throat/nasal swabs, blood samples should be collected for detecting Immunoglobulin G (IgG) antibodies for ELISA testing. In subsequent rounds, IgG ELISA-based testing of serum samples will replace RT-PCR based testing for surveillance purpose," according to the official documents.

Findings in Ahmedabad

A survey conducted by the Ahmedabad Municipal Corporation (AMC) showed that 23.24 per cent of samples had antibodies against coronavirus. Another survey, conducted to find the prevalence of antibodies among those who have recovered from COVID-19, revealed that 40 per cent of such people did not have the antibodies.

The latest findings indicate that the city is far from developing a herd immunity which requires 70-80 per cent of population developing antibodies, Solanki said. "The 5.6 per cent increase in positivity shows that multi-dimensional steps taken by the AMC have yielded results in controlling the spread of the virus," he said. "23.24 per cent positivity shows that we are far from developing any herd immunity, for which the positivity should be 70-80 per cent," he said.

Indore results

In a survey conducted in Indore in early August, 7.72 per cent were found to have developed antibodies to COVID-19. Indore is the worst-hit district in Madhya Pradesh in terms of the number of coronavirus positive cases and fatalities. "Blood samples of 7,103 people aged above one year were taken under the survey that was carried out from August 11 to 23 with the help of the National Centre for Disease Control," Indore Divisional Commissioner Pawan Kumar Sharma said. Of these, 548 samples, which is 7.72 per cent, were found to be having antibodies to SARS-CoV2. Most importantly, people in the age group of 45 to 60 had maximum antibodies, as per the survey. 

Punjab results

Antibodies against the COVID-19 infection have been found in 27.7 per cent of people in Punjab's containment zones. This was Punjab's first exclusive survey and was conducted in five containment zones from August 1 to 17 among a systematically selected random sample population of 1,250 people. "A total of 27.7 per cent of the people in Punjab's containment zones were found to be positive for COVID antibodies, indicating that they have already been infected and have recovered from the coronavirus," as per the serosurvey's findings. The findings showed that the SARS-CoV2 (COVID-19) antibodies' prevalence in the containment zones to be the highest at 40 per cent in Amritsar district, followed by 35.6 per cent in Ludhiana, 33.2 per cent in SAS Nagar, 19.2 per cent in Patiala and 10.8 per cent in Jalandhar district.

Pune results

Released mid-August, Pune's first serosurvey has found that 51.5 per cent of the 1,644 respondents from five highly-affected wards in the city have seroprevalence of antibodies against SARS-CoV-2, the virus which causes COVID-19 infection. A serological survey is done to detect the presence of specific antibodies, and is used to assess the prevalence of a disease in the population. The survey also indicated a higher prevalence of seropositivity in users of shared toilets (62.3 per cent) compared to users of independent toilets (45.3 per cent).

Delhi results

The second round of monthly sero-prevalence survey in the national capital began on Tuesday, with authorities gearing up for the exercise to further analyse the COVID-19 situation in Delhi. The sample size this time is likely to be about 17,000.  The last sero-prevalence survey was held from August 1 to 7, according to which antibodies against COVID-19 infection were found in 29.1 per cent of people surveyed in the national capital.

Delhi Health Minister Satyendar Jain while announcing the results of the August survey earlier this month had told reporters that 15,000 representative samples were taken from 11 districts, and that the next exercise was to begin from September 1. The minister had also said that prevalence of antibodies found in men was 28.3 per cent, and 32.2 per cent in women. Sero-prevalence of antibodies found in age groups of less than 18 was 34.7 per cent, in 18-50 years 28.5 per cent, and 31.2 per cent in 50 years and above. The sources said the September round of survey will be held over five days, and also likely to follow similar protocols as in the previous exercise.

The exercise was undertaken in the first week of August for a comprehensive assessment of the COVID-19 situation in Delhi and to formulate strategies based on its findings.

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