Denouncing reports on achieving herd immunity as well as community transmission among citizens, the Ahmedabad Municipal Corporation (AMC) released its sero survey finding showing 17.61 per cent having IgG antibodies and “ much less level of immunity for Covid-19”. It also said a majority of the population is still “susceptible”.
The municipal body findings has also refuted Indian Council of Medical Research (ICMR) reports that was not made public on the sero survey done in May during lockdown the containment areas that indicated 49 per cent seropositivity, which were cited by a section of the media.
Touted as the “biggest ever” survey conducted in the world in terms of sample population ratio, AMC’s serological study was carried out covering 30,054 samples, nearly 0.5 per cent of the city’s population, the sample size being 60 times larger than ICMR study.
Quoting ICMR Director-General Dr Balram Bhargav, the report states, “The AMC study states that the ICMR study has not been declared officially,
ICMR study was conducted with only 496 samples taken from containment zones only during lockdown in early May… ICMR study shows a ratio of 79 per million population as against AMC study of 4,770 per million population, therefore, it is not at all representative of actual situation and cannot be relied upon for any conclusion. The sample size of present study is 60 times larger than ICMR study”.
Dr Bhavin Solanki, incharge Medical Officer of Health, AMC, who is also one of the 10 authors of the study, told The Indian Express, “The sero positivity found among residents is very less compared to the level of immunity required for the effective control of any viral disease, which is generally beyond 80 per cent… also the herd is not immune and is still largely susceptible.”
So, a majority of the total population is not yet infected and has not yet developed immunity, he added.
Dr Dileep Mavalankar, director, Indian Institute of Public Health (IIPH), Gandhinagar, who is also among the Covid expert panel of the state government, said, “When we say that 50 per cent of people are susceptible of transmission, the sero positivity rate to develop herd immunity declines from 80 per cent. If that is not the case, how will one justify that in a family of five or six one gets infected and remaining other family members do not. It is not that they wear masks at home or follow social distancing. So there is already some kind of immunity there.”
He also questioned why there were no cases from walled city areas now despite people not wearing masks or following social distancing norms and added, “A lot is yet to be understood.”
On Mavalankar’s questions, Dr Solanki said, “Various factors play role in this. In a family, the viral load might not be high to infect others. Immune system of other family members and co-morbid conditions or antibody developed also have a role.”
Drawing parallels with the studies conducted in the other parts of world, especially Spain, Switzerland and the US, the report states that this study is very much in the line with similar studies… the conclusion is also the same that there is widespread infection and countries are at constant risk, it states.
“The herd is still susceptible to get new infection so we cannot rely on herd immunity but need to rely on preventive measures till a vaccine is available,” Dr Solanki added.
A field level cross-sectional serological study was carried out in the city covering around 30,054 samples at its 75 Urban Health Centers (UHCs) across 48 wards and seven zones from June 16 to July 11, 2020. In cross-sectional serological studies, a single blood sample is drawn from members of the population and tested for the presence of high levels of antibodies to the virus.
However, as a word of caution, the report states that some results may be pointing towards difference in the level of positivity, which might be indicating a vanishing protection over a period of time and that the immunity against Covid-19 may not be long-lasting.
Seropositivity is 17.61 per cent with 5,263 positives out of 29,891 samples. Seropositivity among females is 17.98 per cent and males is 17.29 per cent.
Seropositivity in health workers is 13.64 per cent against 18.71 per cent among general population.
Zone-wise seropositivity range from 6.43 per cent to 28.43 per cent, Central zone with the highest.
The age group wise seropositivity ranges between 15.49% to 22.22%.
Seropositivity of all the zones was higher in second and third week of study as compared to the first week.
Day wise positivity trend during the study period shows slightly increasing trend, suggesting an increase in seropositivity with increasing cumulative cases.
While the test conducted is more in the younger age group, test positivity is more in later age groups.
Among the close contacts, non-health care workers (HCWs) have higher positivity as compared to the health care workers.
Even among health care workers, those working on the field have higher positivity (8.98%) as compared to HCW working at the hospitals.
The positivity at non-Covid hospitals is lower than that of Covid hospitals.
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