The novel coronavirus infection has spread rapidly since it was first identified in Wuhan, China in December 2019. One reason it has been difficult to contain COVID-19, scientists believe, is that people who are infected with the virus but show no symptoms spread the virus without realizing it.

A review of studies of people with asymptomatic novel coronavirus infection published recently in Annals of Internal Medicine has found that almost half of those infected with COVID-19 who are asymptomatic may be able to spread the virus for far longer than first thought.

Contact tracing confirmed several new infections identified during the second sampling had been caused by exposure to asymptomatic people.

This matters because those who are do not look or feel sick are more likely to interact with others, so the fact people can be asymptomatic for an extended period might explain the rapid spread of the virus across the globe.

Looking at data from 16 different studies that tested for novel coronavirus infection using a nasal swab, researchers from Scripps Translational Research Institute found that asymptomatic people accounted for about 40 to 45 percent of novel coronavirus infections. This suggests that not only is more widespread testing needed, but testing programs must include those people who show no signs of COVID-19. As the researchers say, “If asymptomatic transmission is indeed common, testing only those with symptoms would be folly.”

Those who are asymptomatic can infect others for at least two weeks, and possibly longer. When residents of Vo’, Italy were tested for novel coronavirus 14 days apart, about 41 percent and 45 percent of each sample who tested positive were asymptomatic. Those who were without symptoms remained asymptomatic during the two-week period between samplings. However, contact tracing confirmed several new infections identified during the second sampling had been caused by exposure to asymptomatic people.

The first case of novel coronavirus aboard the U.S.S. Theodore Roosevelt was identified on March 22, 2020. As of April 24, about 17 percent of the ship’s crew had tested positive, and about 60 percent of those who tested positive were asymptomatic. Even after a 14-day quarantine, many of the asymptomatic crew members still tested positive.

It's worth keeping in mind that these are all early studies of the current pandemic; the ideal study of asymptomatic novel coronavirus infection has not yet been done, the researchers say. Such a study would need to include a large, representative sample of the general population, and data must be collected over a long enough period to distinguish between persons who are asymptomatic and those who are presymptomatic.

On another ship, the Diamond Princess, CT or computed tomography scans found subclinical abnormalities in the lungs of 54 percent of the 76 asymptomatic passengers on board. Future studies will be needed to confirm this finding, which the researchers call “disturbing.”

The numbers of asymptomatic persons and the fact COVID-19 is deadly in some and relatively mild in others had made it difficult to accurately evaluate the effectiveness of medical intervention and public health measures, such as social distancing and wearing face masks.

The COVID-19 pandemic remains an emerging crisis. With information and results constantly being updated, the smartest plan is to do whatever you can to protect your health, even if it seems like things are getting better.