Post by Admin on Apr 1, 2021 19:04:52 GMT
Zoonotic origins
COVID-19 is a zoonotic virus. From phylogenetics analyses undertaken with available full
genome sequences, bats appear to be the reservoir of COVID-19 virus, but the intermediate
host(s) has not yet been identified. However, three important areas of work are already
underway in China to inform our understanding of the zoonotic origin of this outbreak.
These include early investigations of cases with symptom onset in Wuhan throughout
December 2019, environmental sampling from the Huanan Wholesale Seafood Market and
other area markets, and the collection of detailed records on the source and type of wildlife
species sold at the Huanan market and the destination of those animals after the market
was closed.
Routes of transmission
COVID-19 is transmitted via droplets and fomites during close unprotected contact between
an infector and infectee. Airborne spread has not been reported for COVID-19 and it is not
believed to be a major driver of transmission based on available evidence; however, it can
be envisaged if certain aerosol-generating procedures are conducted in health care facilities.
Fecal shedding has been demonstrated from some patients, and viable virus has been
identified in a limited number of case reports. However, the fecal-oral route does not
appear to be a driver of COVID-19 transmission; its role and significance for COVID-19
remains to be determined. Viral shedding is discussed in the Technical Findings (Annex C).
Transmission in Wuhan
Early cases identified in Wuhan are believed to be have acquired infection from a zoonotic
source as many reported visiting or working in the Huanan Wholesale Seafood Market. As
of 25 February, an animal source has not yet been identified.
At some point early in the outbreak, some cases generated human-to-human transmission
chains that seeded the subsequent community outbreak prior to the implementation of the
comprehensive control measures that were rolled out in Wuhan. The dynamics likely
approximated mass action and radiated from Wuhan to other parts of Hubei province and
China, which explains a relatively high R0 of 2-2.5.
The cordon sanitaire around Wuhan and neighboring municipalities imposed since 23
January 2020 has effectively prevented further exportation of infected individuals to the
rest of the country.
Transmission in Hubei, other than Wuhan
In the prefectures immediately adjoining Wuhan (Xiaogan, Huanggang, Jingzhou and Ezhou),
transmission is less intense. For other prefectures, due to fewer transport links and human
mobility flows with Wuhan, the dynamics are more closely aligned with those observed in
the other areas of the country. Within Hubei, the implementation of control measures
(including social distancing) has reduced the community force of infection, resulting in the
progressively lower incident reported case counts.
Transmission in China outside of Hubei
Given Wuhan’s transport hub status and population movement during the Chinese New
Year (chunyun), infected individuals quickly spread throughout the country, and were
particularly concentrated in cities with the highest volume of traffic with Wuhan. Some of
these imported seeds generated limited human-to-human transmission chains at their
destination.
Given the Wuhan/Hubei experience, a comprehensive set of interventions, including
aggressive case and contact identification, isolation and management and extreme social
distancing, have been implemented to interrupt the chains of transmission nationwide. To
date, most of the recorded cases were imported from or had direct links to Wuhan/Hubei.
Community transmission has been very limited. Most locally generated cases have been
clustered, the majority of which have occurred in households, as summarized above.
Of note, the highly clustered nature of local transmission may explain a relatively high R0 (2-
2.5) in the absence of interventions and low confirmed case counts with intense quarantine
and social distancing measures.
COVID-19 is a zoonotic virus. From phylogenetics analyses undertaken with available full
genome sequences, bats appear to be the reservoir of COVID-19 virus, but the intermediate
host(s) has not yet been identified. However, three important areas of work are already
underway in China to inform our understanding of the zoonotic origin of this outbreak.
These include early investigations of cases with symptom onset in Wuhan throughout
December 2019, environmental sampling from the Huanan Wholesale Seafood Market and
other area markets, and the collection of detailed records on the source and type of wildlife
species sold at the Huanan market and the destination of those animals after the market
was closed.
Routes of transmission
COVID-19 is transmitted via droplets and fomites during close unprotected contact between
an infector and infectee. Airborne spread has not been reported for COVID-19 and it is not
believed to be a major driver of transmission based on available evidence; however, it can
be envisaged if certain aerosol-generating procedures are conducted in health care facilities.
Fecal shedding has been demonstrated from some patients, and viable virus has been
identified in a limited number of case reports. However, the fecal-oral route does not
appear to be a driver of COVID-19 transmission; its role and significance for COVID-19
remains to be determined. Viral shedding is discussed in the Technical Findings (Annex C).
Transmission in Wuhan
Early cases identified in Wuhan are believed to be have acquired infection from a zoonotic
source as many reported visiting or working in the Huanan Wholesale Seafood Market. As
of 25 February, an animal source has not yet been identified.
At some point early in the outbreak, some cases generated human-to-human transmission
chains that seeded the subsequent community outbreak prior to the implementation of the
comprehensive control measures that were rolled out in Wuhan. The dynamics likely
approximated mass action and radiated from Wuhan to other parts of Hubei province and
China, which explains a relatively high R0 of 2-2.5.
The cordon sanitaire around Wuhan and neighboring municipalities imposed since 23
January 2020 has effectively prevented further exportation of infected individuals to the
rest of the country.
Transmission in Hubei, other than Wuhan
In the prefectures immediately adjoining Wuhan (Xiaogan, Huanggang, Jingzhou and Ezhou),
transmission is less intense. For other prefectures, due to fewer transport links and human
mobility flows with Wuhan, the dynamics are more closely aligned with those observed in
the other areas of the country. Within Hubei, the implementation of control measures
(including social distancing) has reduced the community force of infection, resulting in the
progressively lower incident reported case counts.
Transmission in China outside of Hubei
Given Wuhan’s transport hub status and population movement during the Chinese New
Year (chunyun), infected individuals quickly spread throughout the country, and were
particularly concentrated in cities with the highest volume of traffic with Wuhan. Some of
these imported seeds generated limited human-to-human transmission chains at their
destination.
Given the Wuhan/Hubei experience, a comprehensive set of interventions, including
aggressive case and contact identification, isolation and management and extreme social
distancing, have been implemented to interrupt the chains of transmission nationwide. To
date, most of the recorded cases were imported from or had direct links to Wuhan/Hubei.
Community transmission has been very limited. Most locally generated cases have been
clustered, the majority of which have occurred in households, as summarized above.
Of note, the highly clustered nature of local transmission may explain a relatively high R0 (2-
2.5) in the absence of interventions and low confirmed case counts with intense quarantine
and social distancing measures.