At Least A Third Of Coronavirus Infections Are Asymptomatic

The pandemic has corrected several common misconceptions about health, like the assumption that you only catch and spread infectious disease when you seem sick.

At one extreme, the measles virus always reveals signs of infection, whereas at the other end, many of those infected with polio virus show no clear symptoms.

Where does the SARS-CoV-2 coronavirus sit on that scale of causing symptoms?

Researchers have now estimated the proportion of infected people who never develop symptoms of Coronavirus Disease.

The research by Daniel Oran and Eric Topol from Scripps Research Translational Institute in La Jolla, California, involved a systematic review of reports that tested for Covid. Those tests either looked for current viral infection through PCR (polymerase chain reaction) analysis or via past infection, as indicated by antibody testing — the presence of antibodies against the SARS-CoV-2 virus.

Oran and Topol’s review, published in Annals of Internal Medicine, found 61 reports, 43 of which used PCR after collecting nose/mouth swabs, and 18 that had performed antibody testing.

The study aimed to count the number of people who never have symptoms of Covid — asymptomatic cases — and exclude those who initially show no signs but then eventually develop the disease. As it’s only possible to identify the latter — presymptomatic cases — in retrospect, the study only considered reports with a follow-up period that tracked whether Covid appeared later.

Among the reports, the best data came from large-scale surveys in England and Spain, which tested antibodies in over 365,000 and 61,000 patients respectively. Results from those two surveys were almost identical: 32.4% of England’s cases were asymptomatic, while Spain’s stats were 33%.

The review therefore suggests that at least one-third of Coronavirus infections are asymptomatic.

There are a few caveats. False positive results from PCR and antibody tests can lead to an overestimate of Covid cases, for example, wheres false negatives mean an underestimate, so you assume everything evens-out. A survey will also rely on accurate self-reporting by its participants, which requires people to try and recall whether they experienced any symptoms weeks or even months earlier.

It’s also important to note that the results don’t mean that only a third of the infected individuals you might encounter won’t show symptoms. Remember that many people are asymptomatic during early infection but turn out to be presymptomatic cases. As a consequence, the proportion of people who are walking around without any apparent symptoms is actually higher than one-third.

A few studies of both asymptomatic and presymptomatic cases and found that those infected people contribute more than 40% to Coronavirus transmission.