28% of COVID-19 cases in India till April 30 asymptomatic: Study

At least 28 per cent of 40,184 people who had tested positive for COVID-19 between January 22 and April 30 in India were asymptomatic, a study found, raising concerns about the novel coronavirus being spread by those who show mild or no symptoms.

A large proportion of both those tested and those positive were asymptomatic contacts, according to the study by the Indian Council of Medical Research (ICMR) scientists along with other collaborators.

Healthcare workers constituted about 5.2 per cent of the total infections, said the study published in the Indian Journal of Medical Research (IJMR).

According to the study, of the 28.1 per cent asymptomatic patients, 25.3 per cent were direct and high-risk contacts of the cases, while 2.8 per cent were healthcare workers in contact with confirmed the case without adequate protection.

“However, the proportion of the asymptomatic infected people could be much more than the 28.1 per cent and this is a cause of concern for us,” Manoj Murhekar, director of the ICMR’s National Institute of Epidemiology and one of the authors of the study, said.

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The findings stated that the proportion of confirmed cases was highest among asymptomatic contacts, two-three-fold higher than among those with severe acute respiratory infection (SARI), those with an international travel history or healthcare workers, Murhekar said.

Between January 22 and April 30, a total of 10,21,518 people were tested for severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2).

Testing increased from about 250 individuals per day in the beginning of March to 50,000 by the end of April. Overall, 40,184 (3.9 per cent) tested positive for the novel coronavirus.
In this study, an analysis of the laboratory surveillance data was done to describe the testing performance and descriptive epidemiology of COVID-19 cases by time, place and person.

The attack rate (per million) by age was highest among those aged 50-69 years (63.3) and was lowest among those under 10 years (6.1).

The attack rate was higher among men as 41.6 of them tested positive, while 24.3 of the women contracted the infection.

Coronavirus cases were reported from 523 of the 736 (71.1 per cent) districts in the country.

States with the highest proportion of districts reporting positive cases included Delhi, Maharashtra, Kerala, Punjab, Haryana, Tamil Nadu, Andhra Pradesh and Gujarat.

States/UTs with the highest test positivity are Maharashtra (10.6%), Delhi (7.8%), Gujarat (6.3%), Madhya Pradesh (6.1%) and West Bengal (5.8%).

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Among the 12,810 cases with reported symptoms at the time of specimen collection, cough and fever were the most commonly-reported symptoms, while around one-third of cases reported sore throat and breathlessness.

Gastrointestinal symptoms such as abdominal pain, nausea, vomiting and diarrhoea were reported by less than five per cent of cases, the study stated.

With implementation of the ICMRs laboratory-based surveillance for the coronavirus, testing was available and accessible, and thus contributed to improved case detection throughout the country, the study underlined.

The network of COVID-19 testing laboratories and testing capacity continues to expand.

“While the cumulative number of cases continues to increase, the growth rate of reported cases has slowed,” it said.

Serosurvery to check community transmission

Meanwhile, according to protocols drafted by researchers from the ICMR and other agencies, the top 10 hotspot cities reporting the highest number of COVID-19 cases in the country will be covered in a serosurvey to estimate the prevalence of community transmission of the infection.

Besides, the survey would also be conducted in 60 districts of 21 states and Union Territories categorized into four strata — zero, low, medium and high — according to the reported COVID cases per million population, as per the protocols for community-based surveillance published in the IJMR.

Among the top 10 cities reporting the maximum number of cases are Mumbai, Delhi, Pune, Ahmedabad, Thane, Indore, Jaipur, Chennai and Surat.

The protocols — titled National sero-surveillance to monitor the trend of SARS-CoV-2 infection transmission in India: Protocol for community-based surveillance — for carrying out the survey was published in the IJMR on Sunday.

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The ICMR said in a statement that the household-level cross-sectional survey will cover 24,000 adults distributed equally across four strata of districts — 15 districts from each category.

Sero-survey involves testing of blood serum of individuals to monitor trends in prevalence of the SARS-CoV2 at district level.

“The trend of seropositivity will also be looked at to monitor the community-level transmission. Pooled seroprevalence from the group of districts for each of the four strata will be used to estimate the prevalence of COVID-19 infection in the country,” according to the protocols.

The initial survey would serve as a baseline to determine the sero prevalence of SARS-CoV-2 infection in the community and in high-burden cities as well, while the subsequent rounds would help to monitor the trends of infection in the community.

The findings of the survey will also guide the strategy for making decisions related to lockdown options at district level, the document said.

“The objectives of this serosurveillance are to estimate and monitor the trend of seroprevalence for SARS-CoV-2 infection in the general population and high-burden cities, determine the socio-demographic risk factors for the coronavirus infection and delineate the geographical spread of the infection in the general population and hotspot cities,” the ICMR said.

The ELISA-based antibody tests would be used and data collection will be done using a mobile-based application.

“The results of the survey will provide information about spread of SARS-CoV-2 infection in different parts of the country. As a part of the study, people from red, orange and green zones in a district will be tested for COVID-19 at random to check whether they have developed antibodies against the infection even though they remained asymptomatic or showed mild symptoms,” an expert associated with the survey had said.

The survey will involve collection of venous blood samples from 400 randomly selected individuals (one per household) from 10 clusters in each district, the official had said.

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Explaining the procedures to be followed, the document stated that the study team will visit the randomly selected households and brief them about the survey objectives and process involved.

After obtaining written consent, information on basic demographic details, exposure history to laboratory-confirmed COVID-19 cases, symptoms suggestive of COVID-19 in the preceding one month and clinical history will be recorded, it said.

A nationally representative serosurvey with multistage sampling design will be useful to identify the most affected areas and provide crucial data to develop epidemiological models to predict the trajectory of COVID-19, the document said.

Instead of waiting for the end of the epidemic, repeated serosurvey’s carried out at regular intervals can be a useful tool to monitor the epidemic precisely, it said.

The survey is being conducted by the ICMR, New Delhi, in collaboration with the Department of Health, the National Centre for Disease Control with support from state health departments and key stakeholders, including the WHO.

The survey is coordinated by ICMR’s National Institute of Epidemiology (NIE) and National Institute of Research in Tuberculosis (NIRT), Chennai.

(With inputs from agencies)