A third serosurvey conducted by the Ahmedabad Municipal Corporation post-October 15 has concluded that despite a large number of Covid-19 cases reported in the period after the second sero survey in August, there was a marginal increase in seropositivity in the general population indicating that the neutralising antibodies’ stayed only for three to five months.
The study also showed that only about half of those who were infected with Covid-19 showed the presence of antibodies.
Interestingly, following the second serosurvey, where an increase of average seropositivity by 5.63 per cent was seen compared to the first serosurvey, AMC had claimed that “The increase in the positivity of 5.63 % after 1.5 months of the earlier study and that too in unlocking period is very less. This confirms that the effective control by AMC overspread of COVID infection through various measures like aggressive testing, proactive identification of cases and their treatment, effective containment etc. has paid rich dividends.”
However, the third serosurvey saw only around one per cent increase in seropositivity compared to the August study.
With 4,117 samples positive for IgG antibodies from a total of 17,009 general population samples from Ahmedabad, the seroprevalence for antibodies averaged at around 24.20 per cent.
An additional 5,483 samples were also taken of health care workers working at non-Covid-19 hospitals or urban primary healthcare centres or as fieldworkers, of contacts of confirmed COVID 19 cases, both family and exposure-related contact, and confirmed cases. Overall seropositivity across the total 22,492 samples stood at 26.41 per cent.
While in the case of earlier two seroprevalence studies, the AMC had emphatically stated that the data does not indicate herd immunity, the third study has instead concluded that “definite comment on the status of herd immunity status cannot be derived on the basis of the available data.”
Overall, among the 1,708 confirmed Covid-19 cases sampled as part of the study, only 54 per cent were detected positive for antibodies.
The AMC study hypothesized that this may be “associated with the time duration from their diagnosis of Covid-19 as they might have lost their antibodies over the long time-gap or it may be on account of the severity of their clinical illness.”
The study notes that antibodies can take approximately one to three weeks to develop, after an infection. As per one of the researchers of the study, the absence of antibodies can be due to low viral load, comorbid conditions, compromised or low immunity, among other reasons.
The study also found that the seropositivity among those who were infected with Covid-19 was “generally stable between 50 to 55% for up to 4 months” and for cases wherein they had completed five to six months since their Covid-19 diagnosis, seropositivity was higher at over 60 per cent.
While the study notes that age group-wise seropositivity shows higher seropositivity among children and elderly, it is pertinent to note that samples taken for 0-9 years (114) as well as for age groups of 80-89 years (81) and 90-99 years (4) were small compared to that taken for other age groups.
As a result, the confidence interval yielded wide ranges, if statistical accuracy has to be considered. An AMC official admitted that the study is meant to serve as a guideline for the civic body as well as the general public and is not necessarily an exhaustive conclusion, “which can only be drawn with larger sampling.”
On the other hand large sampled age groups – from 10-19, 20-29,30-39, 40-49, 50-59 and 60-69 years – where sample size ranged between 1,350 and 4,800, seropositivity results ranged between 21 and 29 per cent.
Looking at the study results zone-wise, of the total 17,009 general population samples, the maximum samples were taken from the north zone (3,001) and the least from the south-west zone (1,449). Both zones had reported a rise in cases in October and November. The two zones reported seropositivity of 27 per cent and nearly 19 per cent respectively.
Northwest zone on the other hand saw about a fifth of the 2,200 samples taken from the zone tested positive for antibodies.
Notably, AMC’s study notes that the zones that saw a high number of cases – central zone and south zone – in the initial days, showed the highest seroprevalence.
This was followed by zones with a high number of cases during the middle phase of the pandemic which were north and east zones and last to follow are the zones which reported a high number of cases in the recent past, that is west, north-west and south-west zones.
The seroprevalence among health care workers sampled shows lower seropositivity as compared to the general population. This may be due to the fact that there are a high number of cases among the general population, implying that the Covid19 incidence among non-Covid health workers was lower, in proportionate comparison.
“Moreover, the health care workers included in our study are not those from Covid hospital but from the field area/UPHC. So, they are at low risk compared to those posted at Covid hospitals and they are also better protected,” stated the survey.
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