Monkeypox, COVID, and polio: These three very different diseases have been dominating news cycles recently, but they share at least one common characteristic: some people can become infected – and in turn infect others – while showing no symptoms.
In 1883, the famous bacteriologist Friedrich Loeffler (1852-1915) recognized that an individual’s asymptomatic carriage of bacteria could lead to diphtheria in others.
“Typhoid Mary” is perhaps the quintessential example of asymptomatic transmission of infections leading to illness and death. At the turn of the 20th century, young Mary Mallon emigrated from Ireland to New York, where she soon became a cook for wealthy Manhattan families.
George Soper, a sanitary engineer, was hired by a stricken family to investigate. After epidemiologic study, he suspected that Mary was a carrier of Salmonella typhi, the bacterial cause of typhoid fever. He persuaded the New York Department of Health to test her – against her will – for infection. After her stool was found to test positive for Salmonella, Mary was forcibly moved to North Brother Island, where she remained largely isolated from others for the next 2 years. In 1910, she was released by a new commissioner after promising not to work as a cook.
However, working under an assumed name, Mary resumed cooking at the Sloane Hospital for Women in Manhattan. Over the next 3 months, at least 25 staff members became ill. Having been found out, Mary was again exiled to the island, where she spent the rest of her life. She died in 1938 after having infected at least 122 people, five of whom died.
COVID
Asymptomatic infections are primary drivers of COVID. Earlier in the pandemic, a meta-analysis suggested a 40% rate of asymptomatic infections, although some early reports arrived at lower estimates. A 2021 JAMA Network Open modeling study indicated a 60% rate.
Those rates are changing with the Omicron variants, of which even more cases are asymptomatic. Is this from a mutation in the virus? Some suggest that it is most likely attributable to prior vaccination resulting in boosted immunity and infections being milder. Of concern is that, although people may be asymptomatic, they still have the same viral load in their nose and can readily transmit infection.
Vincent Racaniello, PhD, a professor of virology at Columbia University in New York, told this news organization that “SARS-CoV-2 COVID is so effective at transmitting because it does this asymptomatic transmission. And so you’re out and about; you have no idea that you’re infected. You’re effectively doing what we call community transmission.”
This distinguishes SARS-CoV-2 from SARS-CoV-1. SARS-CoV-1 – which caused the SARS epidemic in 2002–2004 – had very little asymptomatic shedding. With COVID, on the other hand, “A lot of people are infected but never transmit,” Dr. Racaniello added. “I think 80% of transmissions are done by 20% of infected people because those are the ones who are shedding the most virus.”