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Patients start infection spread before symptoms set in: Study
Infectiousness declined significantly eight days after symptoms appear, but 44% people got infected by people with no visible signs of illness, according to a study from Hong Kong published in Nature Medicine on April 15.
People with the coronavirus disease (Covid-19) start infecting others two to three days before they develop symptoms of fever and cough, according to a study from China which suggests effective outbreak containment must include contact tracing of people who have been exposed to positive cases two to three days before their symptoms first appeared.
Infectiousness declined significantly eight days after symptoms appear, but 44% people got infected by people with no visible signs of illness, according to a study from Hong Kong published in Nature Medicine on April 15.
Earlier studies have found presymptomatic transmission at a higher 48% in Singapore and 62% in Tianjin, China, which led epidemiologists to focus on undetected cases with mild or no symptoms who are not screened or tested for Covid-19.
The new study from China found contagiousness started 2.3 days before symptoms began and peaked 0.7 days before symptoms appeared. Virus production was the highest at the start of the symptoms and started slowly declining after one week.
“Since India’s testing is primarily based on symptomatic positive cases, it is imperative now to widen the criteria for testing by systematic sampling in clusters and hotspots. Tracking the disease will be a huge challenge if half the population of confirmed cases has transmitted in the pre-symptomatic phase,” said Dr Ramana Dhara, professor of public health, Indian Institute of Public Health, Hyderabad.
The most common signs and symptoms of Covid-19 are cough, fever or chills, and shortness of breath, with around one in four people developing gastrointestinal symptoms such as diarrhoea, nausea and vomiting, the study found.
Contact tracing must include pre-symptomatic contacts. “For a reproductive number of 2.5, contact tracing and isolation alone are less likely to be successful if more than 30% of transmission occurred before symptom onset, unless more than 90% of the contacts can be traced. This is more likely to be achievable if the definition of contacts covers two to three days prior to symptom onset of the index case, as has been done in Hong Kong and mainland China since late February,” said study lead author, Eric HY Lau, from the World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, University of Hong Kong, Hong Kong.
Covid-19 is highly infectious, with a basic reproductive number of 2.2 to 2.5. Patients produce thousands to millions of viruses in their noses and throats, about 1,000 times as much virus as produced in Sars patients, till eight days after infection. Even people who don’t develop symptoms can produce enough virus to infect others, according to a small study from Germany published last month in medRxiv, which prints yet to be peer-reviewed studies.
A modelling study done using clinical data from 26,000 lab-confirmed cases in Wuhan, China, said there were 37,400 undetected infections in the province, with most of these unreported cases being people with mild or no symptoms but who were contagious. The study, done in China and the US, was published in medRxiv in early March.
“An increasing number of infected people cannot be linked to known Covid-19 cases or travel to hotspots, which indicates asymptomatic transmission. Presymptomatic transmission reduces the effectiveness of control measures, such as isolation, contact tracing and quarantine, which is why we need hand hygiene, use of face masks by everyone, social isolation and lockdowns to stop the spread,” said a union ministry of health and family welfare official, requesting anonymity.
But lockdowns are not enough. “In India, housing is also an important factor because of the challenge in maintaining physical distance. Mask use is mandatory in Telangana, for example, but I have seen many people not wearing it or maintaining distance in streets, markets etc,” said Dr Dhara.
Countries with active case finding have a higher proportion of presymptomatic transmission because even asymptomatic people are tested and the positive cases and their contacts are quickly isolated and quarantined, which helps prevent spread.
While Covid-19 infectiousness declines significantly after eight days of the symptom appearing, the virus can be detected for a median of 20 days in some patients, and up to 37 days among survivors. “The results support our findings that the infectiousness profile may more closely resemble that of influenza than of Sars,” said the study.
