Mary Mallon was an Irish-born cook who catered to the finest families in New York City. But as well as serving food, she gave her hosts something they did not want or expect: typhoid fever. In the early years of the twentieth century she infected dozens of people in the families she served. As the families fell ill, a typhoid researcher was hired to investigate, and he pinned the blame on Mary. She was found to be a carrier of the bacteria that causes typhoid fever and eventually confined on North Brother Island, to stop her infecting others, eventually becoming something of a celebrity. For her entire life she showed no symptoms, and Typhoid Mary has become a byword for asymptomatic disease carriers, who unwittingly spread infection.

According to a new study from the Princess Diamond cruise ship outbreak, many of us could be a Typhoid Mary, or Coronavirus Carol, spreading COVID-19 but with no signs of disease. Japanese researchers tracked the outbreak through the cruise ship and showed that 17.9% of those positive for SARS-CoV-2, almost one person in five, showed no symptoms. 

As well as the findings from the Princess Diamond, a small study of the outbreak in China showed that viral load was high at the start of the infection, and just as high in those with symptoms as those with none. Supporting this, a cluster of cases was shown to be due to a family member who clearly infected others before they had any symptoms, when they were pre-symptomatic. This shows that the virus can be spread before people have symptoms, as is the case with influenza, but also by those who never develop symptoms. This has implications for the spread of disease, the policy of testing and social distancing, and the chances of severe disease and death.

Given the role of those with no symptoms, only two options are available to slow the COVID-19 outbreak until anti-virals or a vaccine are effective: social distancing of the entire population; or testing of every suspected COVID-19 case, including those with mild symptoms and their contacts, to identify asymptomatic carriers. Social distancing of the entire population could be termed the European strategy, and is in place in Italy, Germany, France, the UK, and many others, with mixed success. 

In contrast, mass testing of all cases and contacts is being used with great success in Taiwan, Hong Kong, Singapore, and South Korea. All of these countries in Asia are well prepared for an infectious disease outbreak, due to past skirmishes with SARS and avian influenza. China started with strict social distancing, and heavily influenced European policy makers, but now that it has cleared the initial outbreak they will most likely shift to the successful Asian strategy - identifying mild and asymptomatic infections and their contacts in order to manage imported cases.

The importance of asymptomatic carriers also forces us to adjust how deadly this virus is. If 18% have no symptoms, then many people with SARS-CoV-2 are not currently being counted in any of the official statistics. Italy, the UK, and many other countries are only testing those with severe disease, and so are excluding both those with mild symptoms (which is over 80%) and also those with no symptoms at all (around 18%). This may help explain why Germany (which is testing all suspected cases, and contacts) seems to have a lower proportion of fatal cases than Italy (which is only testing severe cases).

We won't know how dangerous SARS-CoV-2 is until the much publicised serology tests are rolled out. These detect antibodies to the virus and so can show if you were infected in the past. This will be crucial in finding those who had no symptoms at all, and will allow us to say with some certainty what proportion of infected individuals have no symptoms, mild disease, (a cough and/or fever), severe disease, including pneumonia and the need for ventilation, or fatal disease. 

SARS-CoV-2 is certainly a dangerous virus, and requires unprecedented changes in how we interact with each other. But it is likely less fatal than feared.