Can asymptomatic patients spread COVID-19?

On 7 June, Delhi chief minister Arvind Kejriwal shed some light on the cases emerging in the Indian capital, saying that of the 6,923 cases until then, about 75% of them were asymptomatic or with mild symptoms, stressing the importance for them to isolate at home and monitor their cases. 

New guidelines in India have stressed that patients with mild symptoms or those who are asymptomatic can be under home quarantine, in order to lighten the burden on healthcare workers in the city, and reduce the chances of contracting more severe symptoms of the infection by being at the hospital.

While the 75% figure did share an insight into the way the disease is transmitting in the country, not enough research has been done to study the possibility of asymptomatic people spreading the disease any more or less than those showing symptoms of the disease.

WHO's Dr van Kerkhov's initial statement that asymptomatic patients "rarely" transmit the disease may have been flawed by a long margin, if findings from a new research published in the Annals of Internal Medicine are anything to go by.

In the study titled "Prevalence of Asymptomatic SARS-CoV-2 Infection", conducted by researchers at the Scripps Research Translational Institute, the authors argue that "The silent spread of the virus makes it all the more challenging to control."

The research states that "asymptomatic people account for approximately 40% to 45% of COVID-19 infections, and they can transmit the virus to others for an extended period", even longer than 14 days. 

It is for this reason the researchers believe there was a silent, yet sustained spread of the virus during the initial days of the pandemic, and highlighted the importance of testing. "It's clear that with such a high asymptomatic rate, we need to cast a very wide net, otherwise the virus will continue to evade us."

The authors of the study go on to add that the absence of symptoms does not imply that the respective person will not suffer any harm from the infection. The study done on people who were infected on the Diamond Princess cruise ship in February also revealed an interesting pattern: CT scans done on more than half of the 76 people on board who were asymptomatic showed lung abnormalities that indicated how the SARS-CoV-2 virus affected lung function but without visible symptoms.

The scientists, however, did call for more research to be able to accurately ascertain the clinical significance of their findings. They also added that the lack of longitudinal data, which means to be able to study the same sample group over different periods of time, meant that they couldn't establish the connection or difference between asymptomatic and presymptomatic individuals.

Infected persons who remain asymptomatic play a significant role in the ongoing pandemic, according to various past estimates. The authors of the study published in Annals of Internal Medicine went on to review the available evidence on asymptomatic COVID-19 infection. The authors collected data from 16 different cohorts globally—they gathered their research from several online research publications and data that included healthcare workers, prison inmates, cruise ship travellers and several other subsets. (A cohort study is a study of people who share a common characteristic.)

Data from cruise ship passengers may have been included to test the theory of superspreader events that have significantly raised the number of cases from a single event or gathering. (Read more: Study examines the relationship between COVID-19 and superspreaders)

The study found that of the 3,000-plus prison inmates from four states who had tested positive for COVID-19, as many as 96% were asymptomatic. It went on to add that asymptomatic people can transmit the virus for a longer period of time, even longer than the 14 days that has been widely believed to be the longest gestation period for the virus to infect a human body. Even the viral loads among patients with or without symptoms were similar, but the researchers could not gather if the level of infection in the body is also the same in both cases.

Much like earlier beliefs suggesting that the COVID-19 infection could be around for a few years at a time, the study says that the likelihood of as many 40-45% of asymptomatic patients means that the SARS-CoV-2 virus has much greater potential to spread silently into human populations, and that those with no symptoms are able to transmit the virus for a longer period of time.