A population-based sero-surveillance study carried out in Ahmedabad showed sero-positivity against Covid-19 patients at 23.24 per cent, which is higher than the 17.61 per cent recorded in an earlier study conducted in June 2020.

However, came as a surprise to health officials and authorities is that about 40 per cent of the people, who were once infected with Covid-19, are no more carrying the antibodies.

The study found that of the 1,861 samples from persons who had contracted Covid-19 earlier, only 1,091 or about 60.08 per cent were sero-positive, indicating presence of antibodies.

The authorities fear that, theoretically the loss of antibodies in such a short span of time could lead to second or third waves of infections in Ahmedabad.

Ahmedabad Municipal Corporation conducted the sero-survey during August 15-29 covering 10,310 samples from the general population, other confirmed cases, contacts and frontline healthcare workers. Ahmedabad city has recorded a total of 29,855 cases so far, with 1,677 deaths and 25,190 recoveries, as per AMC data of September 2.

Further risk to contact of confirmed positive cases arises from the fact that 69 per cent of such contacts have shown absence of antibodies. This means that the sero-positivity among contacts of confirmed positive cases is at 31.92 per cent. Of a total of 3,979 samples, as many as 1,268 samples from the contacts of confirmed positive cases have shown sero-positivity.

The sero-positivity in healthcare workers is 23.65 per cent, with only 404 out of 1,708 healthcare workers whose samples were tested, showing sero-positivity. This indicates that healthcare workers too face an increased risk of contracting the infection.

Dr Bhavin Solanki, medical officer of health, AMC, said, “About one-and-a-half months since the last sero-survey, the increase in sero-positivity is only 5.63 per cent, (which) is very less, and that (too) when we were in the unlock period. No herd immunity has yet been developed, as that requires at least 70 per cent positivity. People are still susceptible to new infection. We cannot rely upon this low level of immunity (as is evident from the survey). We have to rely on proven preventive measures till a vaccine is developed and made available to a large number of people. Wearing of masks and social distancing are absolutely necessary.”

He also stated that slow increase in sero-positivity confirms that “the effective control by the AMC over the spread of Covid-19 infection through various measures like aggressive testing, proactive identification of cases and their treatment, effective containment, etc. have paid rich dividends.”