In Tondiarpet, 44.2% of those tested had been exposed to the coronavirus, while Madhavaram reported the least exposure at 7.1%.

Over 20 percent of Chennai people were exposed to coronavirus finds sero-surveyImage for representation/ PTI
Coronavirus Coronavirus Tuesday, September 01, 2020 - 20:18

At least one-fifth of Chennai’s population has been exposed to the SARS-CoV-2 virus that causes COVID-19, according to the preliminary findings of a sero-surveillance study. The sero-survey was carried out by the Greater Chennai Corporation to estimate the exposure of the infection in the city.  

Blood samples from 12,405 individuals from 51 wards across all the 15 zones of the Corporation were collected between July 18 and July 28. These samples were then tested by August 6 for IgG antibodies against the SARS-CoV-2 virus. IgG or Immunoglobulin G antibodies begin appearing by the end of the first week of an individual getting infected by the virus. These antibodies remain in the blood long after the patient recovers, although it is unclear for just how long.  

As per the results of the survey, out of 12,405 persons tested, “2673 individuals had evidence of past SARS-CoV-2, with an unweighted seroprevalance of 21.5%.” Speaking to TNM, Dr Jayaprakash Muliyil, an epidemiologist and former Principal of the Christian Medical College (CMC) in Vellore, says that if these findings are extrapolated to the population of Chennai (71 lakh, as per GCC), it suggests that over 15.26 lakh people have been infected by the coronavirus. The number of confirmed COVID-19 cases reported in Chennai as of September 1 stands at 1,36,697.

The survey also shows huge variation in seroprevalence — members of the population who test positive for the disease based on samples of blood serum — across zones in the city. 

In Tondiarpet, 44.2% of those tested had been exposed to the coronavirus — the highest across the city’s zones. Madhavaram reported the least exposure, with only 7.1% of those tested having antibodies to the virus. 

The other zones that had reported high proportion of individuals with IgG antibodies are Royapuram (34.4%), Thiruvottriyur (31.6%), Teynampet (29.4%), Anna Nagar (25.2%), Manali (24.1%) and Thiru-Vi-Ka Nagar (22.1%). 

In Adyar, 17.3% had been exposed to the virus, followed by Kodambakkam (14.9), Valasarvakkam (14%), Ambattur (13.5%), Alandur (11.1%), Sholinganallur (10.3%) and Perungudi (7.3%). 

Dr K Kolandaswamy, former Director of Public Health in Tamil Nadu, says the findings of the Chennai’s sero-survey indicates effective control measures. 

“The variation is because of the population density. In areas where there is high population density, a higher number of people have been exposed,” says Dr K Kolandaswamy, pointing to the sero-surveillance in Mumbai which showed 57% of its slum population being exposed to the coronavirus, while only 16% in non-slums had antibodies.

Dr Muliyil explains that it is expected for the exposure to the virus to vary from location to location.”Everywhere in India, there is considerable heterogeneity in region-wise, location-wise, sector-wise sero-positivity,” he says, adding, that the reason is economic, with those having to go out to work being at risk of infection, as opposed to those sheltering at home. 

Both Dr Kolandaswamy and Dr Muliyil point out that places like Tondiarpet, which have reported higher exposure, are now seeing a slower transmission of COVID-19. 

“Areas which have reported over 40% exposure to the virus, the current rate of new cases will be low. The number of new cases is low as they are approaching the herd immunity level,” says Dr Muliyil. 

Scientists believe anywhere between 40% to 70% of the community must be immune — either by vaccination or being infected — to achieve herd immunity, which is the point when transmission will no longer be possible. 

With around 80% of Chennai’s population still susceptible to the coronavirus, Dr Kolandaswamy says that it’s important that people do not let down their guard, even as lockdown restrictions ease. 

Noting that acquiring herd immunity is still a long way to go, the former DPH says, “It will take some more time for cases to reduce in Chennai. People still need to be careful. Hand washing, face mask and social distancing is a must. People over 60 and those with comorbidities should avoid going out,” he advises. 

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