May 1, 2020 - Health

The good and bad news about asymptomatic coronavirus cases

A photo of a man in a wheelchair wearing a mask.

Photo: Alexi Rosenfeld/Getty Images

We don’t yet know what proportion of people infected with the coronavirus are asymptomatic, but it’s becoming clear that there’s a large number of them.

Why it matters: The more people that have been asymptomatic carriers of the coronavirus, the lower its fatality rate. But asymptomatic carriers also present unique problems for stopping the virus’s spread, as they likely don't know they have it.

The big picture: Until we can do widespread, reliable antibody testing to determine how many people have had the virus, the best data we have to go off of are one-off studies — which have suggested widely varying rates of asymptomatic carriers.

  • A study earlier this month found that 13.9% of 3,000 New Yorkers tested had signs of the coronavirus, suggesting that about 10 times the number of people who have officially tested positive have had it, per Bloomberg. That means a lot of people either couldn’t get tested, or never knew anything was wrong.
  • Around half of the soldiers on the Theodore Roosevelt aircraft carrier who tested positive for the coronavirus were asymptomatic, per the LA Times. Another study found that about 18% of positive cases on the Diamond Princess cruise ship were asymptomatic.
  • “We don’t know the definitive answer, but it probably is a substantial proportion,” infectious disease expert Anthony Fauci told me. “That is a non-scientifically based estimate, based on these dribs and drabs of information that we get.”

Between the lines: If asymptomatic cases are common, that mathematically increases the likelihood (age and pre-existing conditions aside) that you or I could catch the virus and be completely fine.

  • It also means that a lower percentage of people who get the coronavirus will need hospitalization, which is good news for the health care system.
  • In the darkest of plausible scenarios, where we fail to contain the virus and it spreads relatively unencumbered throughout the U.S., a high asymptomatic rate would translate into a lower death rate — a small comfort.

Yes, but: It also is hugely problematic for efforts to keep the coronavirus from spreading.

  • It could then spread undetected, and if there's already a high number of asymptomatic cases, that means the virus may be more widespread than we thought.
  • It also makes the virus hard to track. “If you have so many asymptomatic people around, it’s going to be much more difficult to get your arms around contact tracing, because you’re going to have so many people who get exposed to someone who is asymptomatic,” Fauci said. That puts extra emphasis on the need to do surveillance testing even among healthy-seeming populations, particularly in places like prisons and nursing homes.

The bottom line: A low number of asymptomatic cases would mean the virus is deadlier than we'd like it to be, while a high number of such cases means it most likely has been more widely transmitted. The uncertainty around that complicates how to manage its spread.

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