Researchers have now indicated that tracking symptoms affiliated with the novel coronavirus (Covid-19) through an app may not be a good predictor of the spread of the disease.

Over the course of 2020, as the Covid-19 has spread across the globe causing extensive illness and economic havoc to communities everywhere, health care providers and the general public have been eager to find a way to identify the illness.

“In the absence of readily available tests, scientists have worked to identify clues to detect those who might have the illness as a way to combat the spread of the virus,” said study authors from Oxford University Press in the UK.

A recent study, published in Nature Medicine journal, suggested that a prediction score combining loss of smell and taste, fatigue, cough, and loss of appetite – collected through an app – was able to prospectively identify people at risk of Covid-19.

For the current findings, published in the journal Family Practice, the research team here compared the main features of the population involved in the Nature Medicine study, and the performance of their score, with data from a cross-sectional study conducted between March 24 and April 29, 2020, and applied the Nature Medicine model to these data.

Applying the probability threshold in the Nature Medicine study to these data indicated that 41 percent of positive tests were false positives, while 17 percent of negative tests were false negatives.

In the Family Practice study, general practitioners referred to most patients because they were complaining of COVID-like symptoms.

“Indeed, nearly half the patients in the sample reported fever (45.4 percent), reflecting the common reason for doctors to refer patients to testing at the time these data were collected,” the authors wrote.

Fever was registered in the app in the Nature Medicine study by only one-third of patients.

In conclusion, while real-time symptom collection through an app seems to be an attractive method to screen for potential infections, and the Nature Medicine study confirms the crucial value of specific symptoms such as loss of smell and taste, the score proposed in the study does not appear to perform well in a primary care population.

“These results confirm the crucial role of laboratory testing in Covid-19 and the need to support research on Covid-19 in primary care populations,” said the letter’s lead author Benoit Tudrej.

In a heartening development for health authorities, a serosurvey conducted in Mumbai has indicated that slum dwellers and women are at the forefront in the march to herd immunity against coronavirus, officials said here on Wednesday.

The ‘Serological Surveillance for SARS-CoV2 Infection’ survey was done by BrihanMumbai Municipal Corporation, along with NITI Aayog and TISS for 12-14 days in the first half of July covering 6,936 people.

Conducted in 3 selected BMC wards, those surveyed comprised 61 percent poor slum dwellers and the rest living in buildings or middle-class housing complexes.

The survey threw up some unexpected and startling results, showing that 57 percent of those tested in the slums was exposed to and developed antibodies against the SARS-CoV-2 Virus, compared to merely 16 percent in housing complexes.

In the surveyed locations of Matunga, Sion, Wadala (F/North), Chembur and Tilak Nagar (M/West), and Dahisar and Mandapeshwar (R/North), it was also revealed that more women had developed antibodies compared to men.

For the slums, the figures thrown up were 59.30 percent women (out of 2,297 tested) and 53.20 percent men (out of 1,937 tested) had developed antibodies against COVID.

In the housing complexes, 16.80 percent of women tested and 14.90 percent of men tested had developed antibodies.

Overall, around 40.50 percent of the people surveyed in Matunga, Chembur, and Dahisar areas were exposed to Covid-19, with Matunga – adjacent to Dharavi slum – having the highest exposure rate of 57.80 percent.

The BMC interprets this high exposure in slums to shared common facilities like toilets, water-taps, high population density, and lack of physical distancing compared to the residents in housing complexes.

With the results suggesting more asymptomatic infections, this higher exposure in slums points to an extremely low death rate in Mumbai – “as low as 0.05 to 0.10 percent” – instead of the existing 5.5 percent.

“So, despite the high exposure to COVID in slums, the survey results indicate that the fatalities are not that high though there could be many factors behind this,” explained a civic official.

The BMC top brass is also startled since none of the people surveyed had taken RT-PCR tests for Covid-19, implying they were asymptomatically infected, and the numbers of such asymptomatic persons are higher than expectations.

The official explained that the presence of antibodies indicated that a person had been infected, fought off the virus, and in the process developed a certain level of resistance to the virus.

The presence of more people with antibodies in a community is considered a positive sign as the virus then has limited scope to spread, implying that measures like wearing masks, and physical distancing were successful in these areas.

At the same time, even the lower presence of antibodies in the housing complexes is considered encouraging as it points to a lesser number of infectees, due to better hygiene, more physical distancing, and other interventions by the BMC, the official added.

“These results will be valuable to learn more about herd immunity. Although it is still unclear what level of prevalence leads to herd immunity, findings indicate that, at least in slums, this could be attained sooner than later, if the immunity exists and persists in a significant proportion of the population,” the BMC said in a statement.

The anti-SARS-CoV2-IgG antibodies were detected using Chemiluminescence assay (CLIA) by the Abbott group, in what is billed as the largest such cross-sectional survey in India.

It was carried out in collaboration with Kasturba Molecular Diagnostic Laboratory, Translational Health Science and Technology Institute, ATE Chandra Foundation, and IDFC Institute.

The development comes a fortnight after the World Health Organization (WHO) acknowledged the success of the Maharashtra government in combating the Covid-19 outbreak in Dharavi.