An infectious person estimated to pass the COVID-19 virus onto two others

February 17, 2020

The number of cases related to the COVID-19, or the Wuhan coronavirus, suggests that the virus can spread via human-to-human transmission through droplets, contact or fomites, similar to the Middle East Respiratory Syndrome (MERS) or the Severe Acute Respiratory Syndrome (SARS).

According to the World Health Organization (WHO), around 16-21% of people with the virus in China became severely ill and roughly 2-3% of those infected have died. A key factor that influences transmission is whether the virus can spread in the absence of symptoms, either during the incubation period (the days before people become visibly ill) or in people who never get sick. 

Chinese officials have claimed transmission had occurred during the incubation period – one asymptomatic child in an infected family tested positive for the virus, though the child never got ill. It is therefore possible for someone to be shedding the virus without having any symptoms. This is concerning because such an individual may never be detected through routine screening and may unknowingly put others at risk.

How contagious a virus is is determined by calculating its reproduction number (R0): the R0 is the average number of other people that one infected person will infect, in a completely non-immune population. Health authorities estimate each infectious person could pass the Wuhan coronavirus onto two others. Different experts have estimated the virus’ R0 could be anywhere from 1.4 to over five; a recent joint research puts the average R0 at 3.28, which exceeds WHO estimates of 1.4 to 2.5 (may be prone to errors).

Transmission of respiratory viruses like COVID-19 are either through large droplets, which is thought to be short-range, and airborne transmission on much smaller particles over longer distances, which is more difficult to control. The amount of Wuhan coronavirus appears to be higher in the lungs than in the nose or throat; and if expelled, could possibly be spread via fine, airborne particles, which are inhaled into the lungs of the recipient. During the SARS outbreak, the virus was considered to be transmitted by contact, over short distances, and even through smaller aerosols over long distances. In Hong Kong, infection was transmitted from one floor of a building to the next.

The continuing surge of cases in China since late January – despite the lockdowns, extended holidays, travel bans and banning of the wildlife trade – could be explained by several factors, such as increased travel for New Year, resulting in the spread of cases around China and globally; more of doctors and nurses coming in from all over China and abroad to help with the response in Wuhan; asymptomatic transmissions that remain undetected; substantial person-to-person transmission; and continued environmental or animal exposure to a source of infection.

While no new countries have reported cases of COVID-19 as of last week, WHO maintains its basic principles to reduce the general risk of transmission, mainly by avoiding close contact with people suffering from acute respiratory infection; practicing frequent hand-washing and good cough etiquette; and avoiding unprotected contact with farm animals. In addition, WHO calls for enhanced infection prevention and control practices within health care facilities, especially in emergency departments.


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