The world as seen other viral epidemics and pandemics recently including SARS, MERS, HIV/AIDS and Ebola.

When SARS broke out in early 2003, it resulted in at least 800 deaths and a significant social and economic cost. Yet, all things considered, the epidemic was relatively rapidly under control and did not turn into a global pandemic.

The transmission characteristics played a crucial factor. Infected individuals were most contagious about 5 to 10 days after the first symptoms appeared. There were also no confirmed cases of asymptomatic individuals that passed on the virus the others.

A team of British researchers including Christophe Fraser and Steven Riley calculated how those viral parameters affect the potential effect of certain mitigation measures. They found that the proportion of transmission that occurs prior to symptom onset is an important parameter for the feasibility of quarantine measures and contact tracing.

If this proportion is very high, neither measure will provide relief. If, on the contrary, the proportion is very low, as in the case of SARS, isolating infected persons is sufficient to reduce the outbreak. When 30-70% of infections happen before or in the absence of symptoms, then contact tracing can have an important effect, at least if its efficiency is high enough.

So when a new virus emerges in early 2020, experts worldwide are holding their breath: how contagious is this virus?

In the first half of January, the number of cases in and around Wuhan would roughly double each week. By the end of the month, the virus had spread to other Chinese provinces, helped by the fact that a lot of Chinese traveled the country to celebrate with the Lunar New Year with family.

By the end of January, we reached nearly 8,000 confirmed cases, and the World Health Organization declared the new virus a "Public Health Emergency of International Concern."

We also turned our eyes to Wuhan. Will the new virus claim as many victims as MERS? Or as SARS? Together with colleagues from the Netherlands, we started modeling with data we obtained from a total of about 200 infections in China that occurred in January and February 2020.

We developed a new method to estimate the generation interval and found that there were about four to five days on average between successive infections. This yielded a higher Rt value than when calculated purely from the serial interval, which is based on the time of symptoms. Moreover, for these data, it appeared that more than half of the infections happened before the first symptoms appeared: 48 to 77% of all infections, according to our calculations. That information would prove crucial to how we would try to reduce the virus worldwide.

Unlike SARS or MERS, where pre-symptomatic spread probably played no significant role, it would not be enough to isolate people with symptoms to contain this new epidemic. Contact tracing and quarantine of all contacts who may be infected would be the only way to prevent further spread of the virus, and since that rarely happens 100% completely, additional measures such as keeping distance will be unavoidable.