Early indications suggest that the disease caused by Covid-19 is typically milder than what was seen with SARS. Most estimates put the case fatality ratio (that is the proportion of known cases who end up dying) at 2% for Covid-19 and around 6-11% for SARS.
Some have suggested that the relative mildness of 2019-nCoV relates to a trade-off that is often seen with respiratory illnesses such as influenza, whereby the more infectious the virus is, the less severe the illness tends to be on an individual level.
This is related to the part of the lungs that virus is more adept at attaching to. If the virus has an affinity for the lower part of the lungs, then the illness tends to be worse for the infected individual. However, it is also less likely to spread to other people as it’s less likely to be breathed, sneezed or coughed out than a virus with an affinity for the upper respiratory tract. Certainly, 2019-nCoV does look to be more infectious than SARS even if the illness it produces is typically milder. In fact, 2019-nCoV has now infected far more people than SARS ever did, which has actually resulted in the higher overall number of deaths.
The frequency and scale of these zoonotic outbreaks (outbreaks that have started because a pathogen has jumped from an animal population into a human one) do look to be increasing.
When thinking about what might be driving this, it’s perhaps useful to think about the process of these kinds of outbreaks taking place in three parts: things that lead to a greater circulation of the pathogen or virus within the animal population; things that lead to greater viral spillover (i.e. the pathogen making the jump from the animal population into the human one); and things that lead the virus to spread between humans more. We know from a range of studies that viruses often jump from animal populations into human ones but then fail to spread much further than the person they originally infected.
Whilst we still do not know the exact mix of factors responsible for the 2019-nCoV outbreak there are some common drivers of zoonotic disease emergence that could be relevant to this situation.
For instance, we know that having a mix of species of animals, especially wild animals, kept close together in stressful and unhygienic conditions, as are sometimes seen in live animal markets, can increase the amount and diversity of viruses circulating between animal species and increase the risk of a virus spilling over into a human host. The conditions in the wet market where this outbreak is believed to have started could therefore have played an important role in triggering this outbreak. Once in the human population, the rapid spread of the virus was likely facilitated by the density of people within the city of Wuhan and the city’s status as an important national and international transport hub.
The frequency and scale of outbreaks of previously rare or unseen pathogens do appear to be increasing and this is likely due to a range of features of globalisation; namely, a rapidly growing, interconnected global population, with a tendency to live in dense cities and with an ever-increasing taste for animal. We are very good at improving our reactive outbreak response infrastructure in the wake of major epidemics; however, we are not so good at learning the lessons relating to outbreak prevention and mitigation. For instance, addressing the drivers of disease emergence I listed above or ensuring a good standard of basic public health infrastructure. Things like ready access to well-resourced primary healthcare facilities can make responding to outbreaks, regardless of the underlying disease, far easier, cheaper and more effective. Simply investing in reactive responses is not going to work in the long-term so we need to start addressing these larger underlying drivers and societal vulnerabilities in a far more meaningful way.
At present its sounds as though the World Health Organization still thinks it is possible that the outbreak might able to be brought under control without significant spread of the disease outside of East Asia. This would mean that countries outside of the region will only have to occasionally deal with an isolated introduced case or two. However, we are already seeing on-going transmission in countries outside of China, for instance in Singapore, and there is a risk that there has already been extensive unobserved transmission of the disease in other regions of the world too. The similarity of the disease caused by 2019-nCoV to other more common respiratory illnesses such as colds and flu, mean that this could well be the case, especially in places that have limited surveillance and testing capacity. If widespread transmission of the virus does begin to occur in other countries, and like I said, this could already be happening, then the most important thing will be to slow the spread of the virus. By doing things like telling people to stay home from work for a couple of weeks if they are sick or have had close contact with a known case (sometime referred to as ‘self-quarantining’) we can limit the number of people sick at a given time. This will hopefully minimise the additional strain put on health systems and limit the number of people unable to work at a given time, avoiding substantial disruption to supply chains and services. The ability to slow the spread of the virus will vary country but country and it is important that we do not neglect low- and middle-income countries, which may not have the resources available to put all the necessary interventions in place and where, with often already struggling health systems, the fallout of such epidemics is often worse.
Tedros Adhanom Ghebreyesus, the Director-General of the World Health Organization, recently suggested that a vaccine might be ready within 18 months. Jeremy Farrar, Director of the Wellcome Trust, suggested one might even be available by February next year, assuming everything goes well. He did also note, however, that despite concerted effort we still don’t have effective vaccines available for any of the other coronaviruses known to infect humans, including the one responsible for SARS or the one responsible for the common cold. With such a long lead time it’s unclear what role a vaccine will play in the response to this outbreak, but it is obviously an important line of enquiry and technology to look into.
There is a lot of work going on at present looking at what kinds of medical interventions might help improve the outcomes for patients, including the development of targeted treatments. The most widely discussed approach to this at present involves using treatments called antiretrovirals normally used to treat people with HIV. Whether this approach will work is yet to be seen, but if it does it could be an important tool in limiting the spread of the virus and mitigating its impact on individuals.