As we move into the late stages of the pandemic, it is important to reflect on the scientific discoveries of the past two years, so that we are better prepared for future virus outbreaks. I believe there are two areas in particular where we can learn from the experience.
The first is understanding that outbreaks are not always easy to identify quickly. Viruses such as Nipah and Ebola cause severe, and therefore obvious, illness in everyone who is infected. The fatality rate is high but transmissibility is low, with an R number – the rate used in epidemiology to measure the reproduction of a virus – of about 2 for Ebola and less than 1 for Nipah. This means that outbreaks can be rapidly identified and contained. In contrast, coronaviruses cause mild, sometimes asymptomatic, infection in the majority of people, but transmissibility is higher. The R number in early 2020 was about 5 – meaning that, on average, each infected person infected five others. Transmission can also occur before symptom onset. The first reports of “pneumonia of unknown cause” in four people that heralded the start of the Sars-CoV-2 outbreak didn’t appear to provide much cause for concern – but those cases were investigated because the 2002 Sars outbreak in the same part of the world had not been forgotten.
