3.4. Susceptible Populations
According to the demographic information of SARS patients, infection occurred in all age groups (the average age was ≦45) [37]. There was a proportional difference between male and female (female predominance) [37], with a male-to-female ratio of 1:1.25 [38]. In addition, hospital staff had a higher risk due to the proximal interactions with large numbers from the infected population. For example, hospital staff accounted for 22% of all cases in Hong Kong and 22.8% in Guangdong [37]. The mortality caused by SARS increased with age (> 64 years) [37], and the overall mortality rate during the outbreak of SARS was estimated at 9.6% [39,40].
Li et al. reported that people who have not been exposed to SARS-CoV-2 are all susceptible to COVID-19 [41]. Among the 8,866 patients who have been confirmed with COVID-19, nearly half of the patients have been aged 50 years or older (47.7%) [36]. The male-to-female ratio is about 2.7:1 [38] and the average incubation period is 5.2 days [42]. However, severe COVID-19 cases and deaths have mostly been in the middle-aged adults and the elderly with long smoking histories or other basic diseases, such as heart disease and hypertension [43,44]. At the time that this paper was been submitted, COVID-19 patients mortality rate was 2.1% [45].