HIVs and SARS-CoVs bear many similarities in terms of cross-species transmission. It is difficult to predict how the ongoing outbreak of SARS-CoV-2 might develop in the coming weeks and months. Unprecedented measures have now been taken to isolate the sources of SARS-CoV-2 infection, to block human-to-human transmission and to protect the susceptible individuals. It remains to be seen whether and to what extent secondary and tertiary spreading will be weakened and prevented by the control measures. Apparently, the intrafamily transmission of SARS-CoV-2 has not been stopped in the epicentre of Wuhan after 23 January 2020, when the city was locked down and human gathering was prohibited. It also remains to be determined what percentage of the general population in Wuhan have been or are being infected by SARS-CoV-2. These are important research questions that should be set as priority. However, as seen in HIV-1, HIV-2, HTLV-3, and HTLV-4, not every animal-to-human transmission event gives rise to a virus that is highly and sustainably transmissible within humans. The transmission of SARS-CoV-2 might be stopped due to the intrinsic characteristics of the virus, the action of human restriction factors, and human intervention measures. Another possibility is that SARS-CoV-2 becomes highly transmissible within humans just like the other four community-acquired HCoVs. Some estimates of the transmission rate expressed as reproductive number (R0) of SARS-CoV-2 fall within the range of 3–4, which is higher than that of SARS-CoV (Table 1). If that can be sustained, SARS-CoV-2 will be well adapted to humans ultimately. It will be fortunate if it also becomes less pathogenic, resembling 229E, OC43, HKU1, and NL63. Plausibly, when they initially crossed species barriers to infect humans decades or centuries ago, 229E, OC43, HKU1, and NL63 might have also caused pandemics in which humans were suffering from more severe respiratory diseases. As mentioned above, one such pandemic recorded at the end of nineteenth century has now been linked to the jumping of OC43 from cattle to humans [26].
Table 1. Comparison between SARS-CoV and SARS-CoV-2.
  SARS-CoV SARS-CoV-2
Virus origin Horseshoe bats as evolutionary reservoir host
Civets as intermediate amplifying host
Unknown reservoir host(s)? Rhinolophus affinis bats as evolutionary reservoir host
Unknown intermediate amplifying host(s)?
Unknown reservoir host(s)?
Entry receptor ACE2 as entry receptor
Both human ACE2 and civet ACE2 capable of supporting SARS-CoV entry
Mouse ACE2 less efficient in supporting entry of SARS-CoV when compared with human ACE2 ACE2 from humans, Rhinolophus sinicus bats, civets and swine as entry receptor [35]
Mouse ACE2 unable to serve as entry receptor [35]
Human-to-human transmission route Droplets in most cases
Close contact with contaminated fomites
Faecal–oral
Aerosols uncommon but possible under special circumstances Droplets in most cases
Close contact with contaminated fomites
Faecal–oral
Aerosols uncommon but possible under special circumstances
Higher attack rate within family clusters
Superspreading events Superspreading events detected in Hong Kong and Beijing [8] Superspreading events suspected as in the Diamond Princess cruise ship.
Clinical presentations Lower respiratory infection
ICU care required in ∼30% patients
ARDS in ∼20% patients
Gastrointestinal and CNS infection Lower respiratory infection [36]
ICU care required in 5–10% patients
ARDS in 5% patients [36]
Gastrointestinal infection
Asymptotic carriers [51]
Case fatality 9.6% worldwide 3.4% worldwide as of 24 February 2020 (4.0% in Hubei Province, China, and 0.84% elsewhere)
Transmissibility R0 = 2a R0 = 3–4b
Interferon antagonists nsp1, nsp3, nsp16, ORF3b, ORF6, M and N proteins nsp1, nsp3, nsp16, ORF3b, ORF6, M and N proteins?
Inflammasome activators ORF3a, ORF8b, and E proteins ORF3a, ORF8, and E proteins?
aR0 is <1 for tertiary and quaternary spreading as well as in the later phase.
bIt remains to be seen as to whether R0 will substantially reduce in tertiary and quaternary spreading as well as in the later phase.