The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes the coronavirus disease (COVID-19), affects people differently. Some people may experience mild to moderate symptoms, while others may require hospitalization due to severe symptoms. Others may develop no symptoms at all.
Called asymptomatic carriers, these individuals can transmit the virus to others without knowing it. Since the pandemic started in December 2019, asymptomatic spread contributed to skyrocketing cases.
Researchers at the Georgetown Center for Global Health Science and Security, Georgetown University in Washington, report that the lack of testing in asymptomatic carriers makes it harder to control SARS-CoV-2 spread.
Transmission without symptoms
The researchers noted in a study published in the journal Science, that asymptomatic cases are those that do not manifest symptoms for the infection duration. In contrast, presymptomatic cases develop symptoms later in the course of infection. These two groups of individuals are crucial drivers of transmission.
Transmission without symptoms poses challenges for identifying the infectious timeline and potential exposures. Further, since they do not know they’re infected, they may likely mingle with others, promoting virus spread. They may also not adhere to masking, distancing, hand hygiene, and staying at home.
Though the prevalence of asymptomatic cases is not accurately reported, early studies showed that these cases accounted for about 30 to 80 percent of infections. Recent evidence reports a rate of asymptomatic cases between 17 and 30 percent.
Assessing secondary transmission
As much as assessing the prevalence of asymptomatic infections is important, it is vital to determine the risk of secondary transmission. Contact tracing relies on identifying cases involving testing people with symptoms. When a person tests positive for SARS-CoV-2, all contacts will be assessed for potential infection.
However, when it comes to asymptomatic cases, it will be challenging to identify close contacts. Surveillance testing to analyze secondary attack rates, the percentage of cases resulting from one infected person in a group, is very difficult.
The true impact of asymptomatic spread
In COVID-19 patients with symptoms, the infectious period starts two days before symptom onset and for several days after. There is also reduced viral shedding within the first week of symptom onset.
In asymptomatic cases, viral shedding kinetics is not well understood. During early infection, people have similar viral loads, regardless of symptom severity. People without symptoms have reduced titers at peak replication and faster viral clearance, making the infectious period shorter.
Hence, measuring the true impact of asymptomatic spread is crucial, though it can be perplexing. Some studies revealed that symptomless individuals were 42 percent less likely to transmit the virus and have lower secondary attack rates. Other studies have noted that despite the shorter infectious period, virus transmissibility is similar for those with presymptomatic or asymptomatic SARS-CoV-2 infection.
The researchers also noted that people with symptoms might have higher secondary attack rates, but they are the ones most likely to undergo testing and self-isolation. People without symptoms may prevent transmission by adhering to public health measures to prevent infection. These include wearing face masks, washing the hands regularly, promoting ventilation, and social distancing.
“With many contagious people experiencing no symptoms and in the absence of robust surveillance testing for asymptomatic or presymptomatic infections, it is critical to maximizing efforts to reduce transmission risk in the community,” the researchers noted in the study.
They added that the lack of testing and isolation of asymptomatic cases could result in surging cases. Governments must impose extensive testing and surveillance to detect even those without symptoms. This way, infected individuals will be isolated appropriately, reducing the risk of viral transmission.
“Until there is widespread implementation of robust surveillance and epidemiological measures that allow us to put out these smokeless fires, the COVID-19 pandemic cannot be fully extinguished,” the team added.
Currently, the number of confirmed SARS-CoV-2 infections has topped 123 million globally, with over 69 million reported as recovered.