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COVID-19 surveillance must also include asymptomatic infections

More than one-third of patients with COVID-19 are asymptomatic, according to a systematic review and meta-analysis of studies reporting laboratory-confirmed infections. Children were more likely to have asymptomatic infections compared to the elderly, while persons with comorbid conditions had greater propensity to be symptomatic.

Researchers from the Yale School of Public Health reviewed data from more than 350 studies, which were published between January 1, 2020 and April 2, 2021. Two separate meta-analyses were conducted. For the study, silent infections were defined as laboratory-confirmed COVID-19 cases that did not exhibit any clinical symptoms at the time of testing. Asymptomatic infections were those that continued to exhibit no clinical symptoms during at least 7 days of follow-up after testing. Presymptomatic cases were persons who developed clinical symptoms after initial testing.

In the first meta-analysis, which included all studies with duration of follow-up sufficient to identify asymptomatic infections, 35.1% infections were found to be truly asymptomatic i.e. they never became clinically symptomatic. The second meta-analysis, which included studies that identified asymptomatic cases at the time of testing and also conducted follow-up to distinguish the presymptomatic stage from asymptomatic infections, the percentage of asymptomatic infections was estimated to be nearly 37%.

Around 42.8%infections had no symptoms at the time of testing; this group included both the asymptomatic and presymptomatic infections. The prevalence of asymptomatic infections was lower in the elderly compared to children; 19.7% vs 46.7%, respectively. Compared to persons with no underlying medical condition, those with comorbidities were much less likely to be asymptomatic.

It is not enough to search for and isolate the symptomatic cases. The true prevalence of infection cannot be estimated if asymptomatic cases are discounted. And, people do not come forward to test themselves, if they have no symptoms.

Evidence has shown that persons with asymptomatic COVID-19 too can transmit the infection. Effective control of the pandemic therefore requires identification of asymptomatic infections also.

India is slowly unlocking and with the imminent reopening of schools and colleges, there is a risk of them becoming superspreaders as this study has shown high prevalence of asymptomatic infections in the younger age group. A focus on detection of symptomatic cases will miss the asymptomatic cases with the consequence that the infection will continue to silently spread in the community. Greater vigilance is required.

Until herd immunity develops and everybody eligible is vaccinated against COVID-19, it is through measures such as wearing masks, physical distancing, hand hygiene, avoiding crowded public places and targeted testing that one can protect themselves, and if infected, reduce the risk of transmission to the community.

(Source: Medscape August 13, 2021 & PNAS August 24, 2021 118 (34) e2109229118)