A large proportion of covid-19 patients’ contacts might have already acquired immunity, according to a study published in the Indian Journal of Public Health. The findings of the study done by Ahmedabad Municipal Corporation, Gujarat on over 4000 contacts of covid-19 positive patients may help the central government further fine tune its vaccination and contact tracing strategy in the backdrop of a second wave spreading across the country.
In significant findings, with 1268 positive for IgG antibodies from 3973 samples, the seropositivity against covid 19 among contacts of cases in Ahmedabad was 31.92%. The seropositivity among family contacts was significantly higher (39.36%) as compared to other contacts (28.72%).
“This trend is seen across all age groups and both the sex groups. The seropositivity has increasing trend with increasing age and is significantly higher among females (35.11%) than males (28.95%). Seropositivity of 31.92% among contacts indicates that a large proportion of contacts have already acquired immunity on account of their contact with the case," the study said.
Generally, during the contact tracing the selection of higher number of other contacts than the family contact supports the fact that generally high number of casual other contacts are recorded for a confirmed case in contrast to limited number of family contacts during the process of contact tracing. Seropositive contacts are those who have developed antibodies against the agent – SARS CoV2 on account of their exposure to the confirmed case of covid 19. The authors argued that these seropositive contacts give an idea about infected but undetected proportion of general population, who might have contributed to the ongoing disease transmission.
The study suggested that the higher risk among family contacts affects both the gender groups equally. Statistically significant higher seropositivity among females for other contacts, require in depth analysis to know the reasons behind this difference, the study said.
However, Ahmedabad Municipal Corporation, Gujarat had already completed one large scale population based serosurveillance for IgG antibodies against SARS CoV 2 Virus with an average positivity of 17.61% at the end of June 2020. The state in association with the Indian Council of Medical Research planned a repeat population based serosurvey to study the covid 19 seropositivity in the second half of August 2020.
“Higher seropositivity among family contacts justifies the risk categorization and testing strategy adopted for the contacts of the cases. This also reaffirms the need for contact tracing strategy for controlling the inevitable spread of pandemic," the authors said.
The study also found that children have slightly higher positivity as compared to young adults with lowest positivity. “This can be explained by the higher susceptibility and lower compliance of preventive measures required for a contact, which leads to subsequent seroconversion," the study said.
Considering all the contacts, young male children and elderly females have higher positivity, the findings of the study said. For elderly, the difference may be due to higher load of household activities by elderly female (particularly in traditional Indian culture) as compared to elderly male contacts during the quarantine, which increases the risk during exposure and affects subsequent seropositivity, the authors said.
Multiple sero surveillance studies have focused on the presence of antibodies against SARS CoV2 in the general population and confirmed cases. The researchers aimed at estimating covid 19 seropositivity among contacts of covid 19 cases and to compare the seropositivity between types of contact for the assessment of differential risk and transmission dynamics.
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