Contrary to the widespread optimism of the mid-20th century, infectious diseases will
never be relegated to history by antimicrobials and vaccines. ‘New’ human pathogens
have emerged at intervals throughout human history but, until recently, emerging infectious
diseases often remained unrecognized, and their causes unknown, long after they had
become widespread. Now, scientists can often characterize emergent pathogens with
astonishing speed, using next-generation sequencing and bioinformatics. In December
2019, in Wuhan, China, only a few days elapsed between collection of respiratory specimens
from patients with viral pneumonia of unknown aetiology and the publication of the
genome of the novel beta-coronavirus (now called SARS-CoV-2) that caused it.
This led to rapid development of a diagnostic polymerase chain reaction (PCR) test
and implementation of targeted containment measures. One might expect these technological
advances, along with lessons from two previous novel coronavirus disease outbreaks—severe
acute respiratory syndrome (SARS), in 2003 and Middle East respiratory syndrome (MERS),
in 2012—to contribute to rapid control of the latest one, COVID-19. But, as Peeri
note in this issue, although there are similarities between SARS-CoV, MERS-CoV and
SARS-CoV-2, there are key differences between the corresponding diseases, the global
milieux into which they have emerged and international responses, that limit the relevance
of lessons from previous outbreaks.
SARS-CoV, MERS CoV and SARS-CoV-2, like most emerging human pathogens, are zoonotic
viruses that have ‘spilled over’ from animals and acquired the ability to spread,
person-to-person, with variable efficiency. SARS-CoV was first implicated in human
disease in China in 2002, linked epidemiologically to wild animal trading and detected
in several wild animal species, before being traced to a reservoir in bats.
Early COVID-19 cases were linked to the Huanan Wholesale Seafood Market, in Wuhan,
which sold a variety of wild animals. No specific animal source was identified, but
the genetic similarity between SARS-CoV-2, SARS-CoV and SARS-like bat coronaviruses,
suggest that bats are the likely reservoir of SARS-COV-2, as they are, also, of MERS-CoV;
the usual intermediary source of MERS, for humans, is domestic camels.
In 2003, the Chinese government was criticised for its delayed reporting of an outbreak
of viral pneumonia in Guangdong province to the World Health Organization (WHO). By
the time the WHO was notified in March 2003, 4 months after the outbreak started,
there were 300 known cases and a Chinese doctor, visiting from Guangzhou, had infected
16 fellow guests in a Hong Kong hotel, thus triggering global dissemination of SARS
to 29 countries.
This contrasts with the more timely notification of a cluster of pneumonia cases in
Wuhan on 31 December 2019. Although, in retrospect, the first cases had occurred in
early December, there was understandable delay in recognizing a small number of serious
respiratory infections as an outbreak, in the midst of a seasonal influenza epidemic.
However, the report closely followed a ‘leak’, on 30 December, when a young Chinese
doctor, Li Wenliang, warned a small group of colleagues by private WeChat message,
about several cases of a SARS-like illness. A few days later he was detained for ‘spreading
rumours’ and made to sign a statement promising to cease ‘illegal activities’. Nevertheless,
Li continued to speak out about the need for greater transparency, until his death
from COVID-19 on 7 February.
On 23 January 2020, the Chinese government introduced control measures designed to
limit spread of the disease, including travel bans to halt the expected mass population
movements during Lunar New Year celebrations, along with social distancing policies,
isolation of cases and quarantine of suspected contacts.
By then, COVID-19 had spread widely within Hubei province, but much less so in other
provinces and very little internationally. Of nearly 43 000 laboratory-confirmed cases,
to 11 February, there were only 466 outside mainland China, mostly in people who had
left Hubei province before travel restrictions were imposed.
Daily reports of new cases in China plateaued between 23 and 27 January and have been
declining since, but by early March case numbers were rising rapidly around the world.
By 9 March, there were more than 114 000 laboratory-confirmed cases, including nearly
34 000 in 112 countries outside mainland China, with several reporting significant
community transmission (https://www.worldometers.info/coronavirus/). The crude case
fatality rate (CFR) was 3.4% but, considering limited testing capacity in many countries,
and the likely substantial numbers of undiagnosed cases, the true CFR is predicted
to be <1%.
Population-based serological surveys will be needed to estimate the true infection
prevalence and CFR.
It was clear early on that COVID-19 was very different from SARS and MERS. Although
presenting symptoms are similar—fever and cough, progressing to pneumonia in severe
cases, with poorer outcomes associated with older age and comorbidities—SARS and MERS
were/are much less transmissible but more likely to be severe or fatal than COVID-19.
The spread of SARS was controlled by July 2003, 7 months after it began, with a total
of 8096 cases and 774 (9.6%) deaths. There have been multiple MERS outbreaks since
2012, mainly in Middle Eastern countries; most have begun with a primary case, acquired
from a camel, followed by secondary person-to-person spread. Altogether, 2494 MERS
cases and 858 (34.4%) deaths, have been reported, worldwide. The high proportion of
healthcare workers affected by SARS (21%
) and MERS (34% of secondary cases
) has not been a feature of COVID-19; until 11 February 11, only 3.8% of confirmed
COVID-29 cases in China and 0.05% of deaths have been in healthcare workers.
Has the global community failed to heed the lessons of previous outbreaks? There are
still many unanswered questions, but current widespread community transmission of
COVID-19, in many countries, indicates that it cannot be eradicated (like SARS) or
limited to modest outbreaks (like MERS). In the absence of a vaccine, public health
measures may, at best, delay transmission. How rapidly it spreads will depend on how
conscientiously members of the public and hospital workers observe well-established
infection prevention and control (IPC) principles—hand hygiene, cough etiquette, social
distancing and, in healthcare settings, isolation of affected patients and appropriate
use of personal protective equipment (PPE).
The rapid dissemination of COVID-19-related information via the internet has been
almost overwhelming and a mixed blessing. Timely scientific, clinical and epidemiological
data are invaluable to policy and decision makers, but an ‘infodemic’ of false information,
especially via social media, has fuelled conspiracy theories, public alarm, stigmatization
of affected communities and escalating economic costs disproportionate to health risks,
significant though they are. Although China’s response was not unduly delayed this
time, all governments would be wise to heed, rather than silence, warnings from astute
clinicians, like Dr Li, who are often the first to recognise a disease cluster. There
is a long way to go and much to learn before the end of COVID-19.
Conflict of interest