By Diana Swift
As COVID-19 cases rise and vaccines are delayed, health authorities continue to look for additional ways to mitigate the spread of the novel coronavirus. [ 1 ]
Now a modeling study estimates that more than half of transmissions come from presymptomatic, never symptomatic, and asymptomatic individuals, indicating that symptom-based detection will have little effect on the spread.
The study by the US Centers for Disease Control and Prevention (CDC), published electronically in JAMA Network Open, concludes that for optimal control, protective measures such as mask use and social distancing should be complemented by strategic testing of potentially exposed but asymptomatic individuals.
“In the absence of effective and widespread use of therapies or vaccines that can shorten or eliminate infectivity, successful control of SARS-CoV-2 cannot rely solely on identifying and isolating symptomatic cases; even if implemented effectively, this strategy would be insufficient,” warned CDC biologist Dr. Michael J. Johansson, Ph.D., and colleagues. “Multiple measures that effectively address the risk of transmission in the absence of symptoms are imperative for controlling SARS-CoV-2.”
According to the authors, the effectiveness of some current transmission prevention efforts has been questioned and is subject to conflicting messages. Therefore, they decided to model the proportion of COVID-19 infections that are likely the result of people who are asymptomatic and may be unknowingly infecting others.
Dr. Jay Butler
"Unfortunately, there is still some skepticism about the value of community-wide mitigation efforts to prevent transmission, such as mask-wearing, distancing, and hand hygiene, particularly for people without symptoms. So we wanted to have a baseline estimate of how much transmission comes from asymptomatic people to underscore the importance of mitigation measures and building immunity through vaccination," corresponding author Dr. Jay C. Butler told Medscape Medical News .
This criterion is especially relevant in the context of the new, more transmissible variant. “It really puts things into a bigger picture and strongly highlights the need to change people’s behavior and the importance of mitigation,” noted Dr. Butler, emphasizing the value of conducting targeted strategic testing in collective settings, schools, and universities, which is already underway.
The analysis
Based on data from several COVID-19 studies from last year, the CDC's analytical model assumes at baseline that infectiousness peaks at the midpoint of symptom onset, and that 30% of infected people never develop symptoms; however, that 75% are just as infectious as those who develop obvious symptoms.
The researchers then model multiple scenarios of presymptomatic and never-symptomatic individuals based on transmission, assuming different incubation and infectious periods, and a variable number of days from the point of infection to the onset of symptoms.
When combined, the models predict that 59% of all transmission would come from asymptomatic transmission: 35% from presymptomatic individuals and 24% from never symptomatic individuals.
The findings complement those of a previous CDC analysis, according to the authors.
The overall proportion of transmission from presymptomatic and never-symptomatic individuals is key to identifying mitigation measures that can control SARS-CoV-2, the authors stated.
For example, if the reproduction number of the infection in a particular environment is 2.0, a reduction in transmission of at least 50% is needed to reduce the reproduction number of the infection below 1.0.
"Given that in some settings the reproduction rate of the infection is likely much higher than 2 and more than half of transmissions may come from people who are asymptomatic at the time of transmission, effective control must mitigate the risk of transmission from people without symptoms," they wrote.
The authors acknowledged that the study applies a simplistic model to a complex and evolving phenomenon, and that the exact proportions of presymptomatic and never-symptomatic transmission and incubation periods are unknown. They also noted that symptoms and transmission appear to vary among different population groups, with older people being more likely than younger people to experience symptoms, according to previous studies. [ 2 ]
"Assume everyone is potentially infected"
Other experts agree that it is important to expand testing to asymptomatic individuals. “Screening for fever and isolating symptomatic individuals is a common-sense approach to help prevent spread, but these measures are by no means adequate, as it has been clearly documented that asymptomatic or presymptomatic people can still spread the virus,” noted Dr. Brett Williams, an infectious disease specialist and assistant professor of medicine at Rush University in Chicago.
“As we saw with the White House super-spreader outbreak, testing does not reliably rule out infection, either because the individual tested has not yet tested positive or because the test is false negative,” added Dr. Williams, who was not involved in the CDC study. He also noted that when prevalence is as high as it is now in the United States, the false-negative rate will be high because a large proportion of those tested will be infected without knowing it.
At their center, all visitors and staff are screened with a thermometer upon entry, and universal mask-wearing has been required since the early days of the pandemic. "Nationally, there have been many cases of outbreaks in hospital break rooms due to staff members having lunch together, and these outbreaks also demonstrate the incompleteness of symptomatic isolation."
For his part, Dr. Frank Esper, a virologist specializing in pediatric infectious diseases at the Cleveland Clinic in Cleveland, Ohio, said that while it has been understood for some time that many infected people will not show symptoms, "the question remains how infectious they are."
Dr. Esper's conclusion from the modeling study is not so much that we need more screening of potentially exposed but asymptomatic people, but that testing symptomatic people and tracing their contacts is not enough.
"We have to continue assuming that everyone is potentially infected, whether they know it or not. And although we have increased our testing capacity to a much greater level than in the first wave, we have to continue wearing masks and social distancing, because simply identifying people who are sick and isolating or quarantining them will not be enough to contain the pandemic."
And while the assumption-based model is useful, it can't tell us "how many asymptomatic people are actually infected," added Dr. Esper, who was not involved in the CDC study.
The specialist also noted that the study's estimates are based on data from early Chinese studies, but the virus has changed since then. The new, more transmissible strain in the United States and elsewhere may involve not only more infections but also a longer presymptomatic stage. "So the CDC study may not actually capture asymptomatic infections."
He also agreed with the authors that when it comes to infection, not all humans are the same. "Older people tend to be more symptomatic and become symptomatic more quickly, so the rate of asymptomatic cases is not the same as among young adults in their 20s and older people."
In conclusion, Dr. David A. Hirschwerk, an infectious disease specialist at Northwell Health in Manhasset, USA, stated that this data supports maintaining the protective measures we have been taking over the past few months.
Dr. Hirschwerk, who was not involved in the analysis, told Medscape Medical News that the data "support the concept that asymptomatic people are an important source of transmission and that we must adhere to mask use and social distancing, particularly indoors. More testing would be better, but it needs to be quick and efficient, and there are many challenges to overcome."