“Assume that everyone is potentially infected”
Other experts agree that expanded testing of asymptomatic individuals is important. “Screening for fever and isolation of symptomatic individuals is a common-sense approach to help prevent spread, but these measures are by no means adequate since it’s been clearly documented that individuals who are either asymptomatic or presymptomatic can still spread the virus,” said Brett Williams, MD, an infectious disease specialist and assistant professor of medicine at Rush University in Chicago.
“As we saw with the White House Rose Garden superspreader outbreak, testing does not reliably exclude infection either because the tested individual has not yet become positive or the test is falsely negative,” Dr. Williams, who was not involved in the CDC study, said in an interview. He further noted that when prevalence is as high as it currently is in the United States, the rate of false negatives will be high because a large proportion of those screened will be unknowingly infected.
At his center, all visitors and staff are screened with a temperature probe on entry, and since the earliest days of the pandemic, universal masking has been required. “Nationally there have been many instances of hospital break room outbreaks because of staff eating lunch together, and these outbreaks also demonstrate the incompleteness of symptomatic isolation,” Dr. Williams said.
For his part, virologist Frank Esper, MD, a pediatric infectious disease specialist at the Cleveland Clinic, said that while it’s been understood for some time that many infected people will not exhibit symptoms, “the question that remains is just how infectious are they?”
Dr. Esper’s takeaway from the modeling study is not so much that we need more screening of possibly exposed but asymptomatic people, but rather testing symptomatic people and tracing their contacts is not enough.
“We need to continue to assume that everyone is potentially infected whether they know it or not. And even though we have ramped up our testing to a much greater capacity than in the first wave, we need to continue to wear masks and socially distance because just identifying people who are sick and isolating or quarantining them is not going to be enough to contain the pandemic.”
And although assumption-based modeling is helpful, it cannot tell us “how many asymptomatic people are actually infected,” said Dr. Esper, who was not involved in the CDC study.
Dr. Esper also pointed out that the study estimates are based on data from early Chinese studies, but the virus has since changed. 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 actually undershoot asymptomatic infections,” he said.
He also agreed with the authors that when it comes to infection, not all humans are equal. “Older people tend to be more symptomatic and become symptomatic more quickly so the asymptomatic rate is not the same across board from young people age 20 to older people.”
The bottom line, said David. A. Hirschwerk, MD, an infectious disease specialist at Northwell Health in Manhasset, N.Y., is that these data support the maintenance of protective measures we’ve been taking over the past months. “They support the concept that asymptomatic people are a significant source of transmission and that we need to adhere to mask wearing and social distancing, particularly indoors,” Dr. Hirschwerk, who was not involved in the analysis, said in an interview. “More testing would be better but it has to be fast and it has to be efficient, and there are a lot of challenges to overcome.”
The study was done as part of the CDC’s coronavirus disease 2019 response and was supported solely by federal base and response funding. The authors and commentators have disclosed no relevant financial relationships.
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