Transmission Dynamics of COVID-19

Analysis of the Transmission Dynamics of COVID-19: An Open Evidence Review.
Jefferson T, Spencer EA, Plüddemann A, Roberts N, Heneghan C.
https://www.cebm.net/evidence-synthesis/transmission-dynamics-of-covid-19/

In the midst of the COVID-19 pandemic, uncertainty on the characteristics of a novel disease reigns.  The explosive nature of COVID-19 transmission, initially shown by the number of new cases and later by admissions and deaths, remains unexplained.

One of the principal uncertainties regards the means by which COVID-19 is transmitted, with special regard to the factors which may accelerate or delay its spread, the mode of transmission, the role of asymptomatic infected people, its speed, the possible interactions with wildlife or livestock, urban or rural environments and population density.

The first part of this Open Evidence Review consists of searching the evidence and posting summaries of all included studies, which are accessible from the searchable table below. As we make more information available, we will define modes of transmission, publish updates with recommendations for policy, and set out a series of hypotheses to be tested by further work.


Published Updates:

10th July 2020: Studying sewage could help solve a coronavirus mystery. Carl Heneghan, Tom Jefferson. The Spectator. www.spectator.co.uk/article/studying-sewage-could-help-solve-a-coronavirus-mystery

3rd July 2020: Environmental Weather Conditions and Influence on Transmission of SARS-CoV-2 Spencer EA, Brassey J, Jefferson T, Heneghan C. www.cebm.net/covid-19/environmental-weather-conditions-and-influence-on-transmission-of-sars-cov-2/

The Open Evidence approach

Our aim is to provide accessible summaries of the evidence of transmission dynamics for COVID-19. To achieve this we provide evidence  summaries that  include :

  • Bottom Line: the take-home points  
  • Evidence Summary: a summary of the main results
  • What did they do? the methods used
  • Study reliability: the points to consider when assessing whether the evidence is affected by bias
  • What else should I consider? Any further  points that add value to the research evidence

We will be undertaking systematic and regular searches for those studies that assess COVID-19 transmission, assessing the quality of the included studies and their implications. The review findings will be indexed on the Oxford Research Archive.

As we go we will set out policy recommendations. To interpret the study summaries we recommend not relying on single studies alone to draw conclusions. We will group the exposures into themes to provide a better understanding of transmission, highlight the key emerging messages, and build a useable, searchable database of the transmission variables for COVID-19.

If you are aware of any evidence that we should include, spot an error, or you just want to comment then email  cebm@phc.ox.ac.uk with the subject  line: transmission dynamics for COVID-19.

All the best the Open Evidence Review Team

Policy recommendations

Investigate the transmission dynamics of COVID-19 in hospitals, nursing homes and dormitories, using classic investigative methods of testing, interview and graphics to reconstruct an outbreak. Read the full recommendation

COVID-19 Dashboards should aid the understanding of outbreak clusters, highlight super spreading events and facilitate mitigation strategies. Read the full recommendation

Evidence explorer

There are 87 papers in this review

Filter papers by Transmission mode


















Title & bottom lineTransmission
mode
Country
1 COVID-19: Theories on the proximal origin of SARS-CoV-2.

Three possible origin theories are proposed and discussed. Not enough data are available to accept or reject any of the hypotheses.

Read the study summary
Orofecal 
2 Gastrointestinal features in COVID-19 and the possibility of faecal transmission.

Gastrointestinal symptoms are common in COVID-19 patients and were observed with increased prevalence as the epidemic progressed in China. SARS-CoV-2 enters GI epithelial cells, and the faeces of COVID-19 patients are potentially infectious.

Read the study summary
Orofecal  Mostly China
3 Concentration and detection of SARS coronavirus in sewage in China

No live SARS-CoV was found in any sewage samples from two hospitals receiving COVID-19 patients. SARS-CoV RNA was detected in sewage concentrates of two hospitals receiving SARS patients prior to disinfection, and occasionally after disinfection.

Read the study summary
Orofecal Waterborne  China
4 Sentinel surveillance of SARS-CoV-2 in wastewater anticipates the occurrence of COVID-19 cases.

SARS-CoV-2 was detected in sewage 41 days before the declaration of the first COVID-19 case in Spain and in frozen samples dating back to 12 March 2019. If confirmed, the results suggest SARS CoV-2 has been around longer than first thought.

Read the study summary
Orofecal Waterborne  Spain
5 COVID-19: Differential occupational risks to healthcare workers from SARS-CoV-2

Among 9,809 healthcare employees in a UK hospital in the south-east, an increased risk of COVID-19 was found in staff in acute medicine, among Black and Asian staff, and porters and cleaners. Protective interventions including PPE appeared to reduce risk among intensive care staff.

Read the study summary
Close contact Person to person  UK
6 COVID-19: TMPRSS2 and TMPRSS4 promote SARS-CoV-2 infection of human small intestinal enterocytes.

In a laboratory study, human enterocytes expressing high ACE2 receptor levels could support infection with SARS-CoV-2. However, the virus is rapidly inactivated in the GI tract, and no infectious virus was obtained from stool samples from COVID-19 patients.

Read the study summary
Orofecal  USA
7 Digestive symptoms in COVID-19 patients with mild disease

Among a group of hospitalised patients with low severity COVID-19, digestive symptoms were present in 57%. Patients with digestive symptoms were more likely to be fecal virus-positive than those with respiratory symptoms.

Read the study summary
Orofecal  China
8 COVID-19: Molecular and serological investigation of 2019-nCoV infected patients

This study indicated that RNA of SARS-CoV-2 maybe shed via multiple bodily routes, and highlights that COVID-19 is found in anal swabs sometimes when oral swabs show no viral RNA.

Read the study summary
Orofecal  China
9 Prevalence of gastrointestinal symptoms and fecal viral shedding in patients with COVID-2019: a systematic review and meta-analysis.

This review reports that the Gastrointestinal tract supports the growth of SARS-CoV-2 to an extent similar to previous SARS infections and that gastric symptoms are frequently experienced in COVID-19.

Read the study summary
Orofecal  N/A
10 COVID-19: Is SARS-CoV-2 Also an enteric pathogen with potential fecal-oral transmission?

Various observational and mechanistic evidence supports that SARS-CoV-2 can infect and be shed from the human gastrointestinal tract.

Read the study summary
Orofecal  N/A
11 COVID-19: Infection of bat and human intestinal organoids by SARS-CoV-2.

SARS-CoV-2 can infect bat intestinal cells, and demonstrated SARS-CoV-2 replication in human intestinal organoids, suggesting that the human intestinal tract might be a transmission route of SARS-CoV-2.

Read the study summary
Orofecal  N/A
12 SARS-CoV-2 in serum, urine, and stool specimens of COVID-19 patients from the Republic of Korea.

SARS-CoV-2 RNA was detected in serum, urine or stool samples in 20% of patients hospitalised with COVID-19. However, the virus could not be isolated from these samples and therefore the risk of transmission via these media is not established. 

Read the study summary
Orofecal Urine  South Korea
13 Infectious SARS-CoV-2 in feces of patient with severe COVID-19.

This case series of 28 hospitalised patients for whom faeces samples were available indicated that infectious virus was present in faeces from two cases who also tested positive for viral RNA by RT-PCR.

Read the study summary
Orofecal  China
14 COVID-19: Household transmission of SARS-CoV-2 in Wuhan.

The observed rate of secondary transmission among household contacts of hospitalised patients with SARS-CoV-2 infection was 30%.

Read the study summary
Close contact Person to person  China
15 A cluster epidemic of COVID-19 in a supermarket in Liaocheng, Shandong Province.

Several cases were apparently transmitted from a supermarket worker.

Read the study summary
Close contact Person to person  China
16 Asymptomatic and paucisymptomatic SARS-CoV-2 infections

This study of two cases and six subsequent infections amongst close family contacts suggests SARS-CoV-2 transmission through asymptomatic or paucisymptomatic infections is possible.

Read the study summary
Close contact Person to person  China
17 COVID-19: Toilets dominate environmental detection of SARS-CoV-2 in a hospital

Of 107 surface samples (37 from toilets, 34 from other surfaces in isolation rooms and 36 from other surfaces outside isolation rooms). Four samples were positive (2 two ward door door-handles, one bathroom toilet toilet-seat cover and one bathroom door door-handle). Three were weakly positive from a bathroom toilet seat, one bathroom washbasin tap lever and one bathroom ceiling exhaust louvre. One of the 46 corridor air samples was weakly positive.

Read the study summary
Orofecal  China, Nanjing
18 The characteristics of household transmission of COVID-19

The secondary attack rate of SARS-CoV-2 within households was 16%. Increased age and being the spouse of the index case increased risk of transmission. Isolation within the home of the index patients, at the onset of symptoms, prevented household transmission.

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Close contact Person to person  China
19 Risk of transmission in COVID-19 among close contacts

2.6% of close contacts of cases contracted COVID-19; almost half were asymptomatic or had  mild infection. The main transmission appeared to take place via household contacts.

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Close contact Person to person  China
20 COVID-19: SARS-CoV-2 infection from contaminated water systems

Based on data from 39 countries, SARS-CoV-2 can remain stable within water for up to 25 Days. Country-specific risk of infection posed by faecal contaminated water is environment-dependent, with water flow and temperature as important variables.

Read the study summary
Orofecal Waterborne  32 Countries
21 COVID-19: Preparation for Quarantine on the Diamond Princess Cruise Ship

Infection rate was high and clinical harm was considerable onboard the cruise ship. A number of operational difficulties hampered ideal isolation and quarantine procedures onboard.

Read the study summary
Close contact Fomites Person to person  Japan
22 COVID-19: High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice – Skagit County, Washington, March 2020

This study of choir practice attendees suggests that transmission of COVID-19 was facilitated by close proximity and physical contact and may have been augmented by the act of singing.

Read the study summary
Aerosol Close contact Fomites Person to person  USA
23 COVID-19: Cluster of Covid-19 cases in the French Alps, 2020

This study highlights the variation possible in transmission between close contacts and also that cases’ clinical experience is highly variable including asymptomatic.

Read the study summary
Close contact Person to person Superspreading  France, England and Spain
24 COVID-19: Community Transmission of SARS-CoV-2 at Two Family Gatherings in Chicago

Gatherings likely to involve physical greetings and prolonged close proximity probably increase transmission of SARS-CoV-2.

Read the study summary
Close contact Person to person  USA
25 COVID-19: Potential for Foodborne transmission of COVID-19

Extensive uncertainty on the ecology of SARS-CoV-2 makes it hard to make specific recommendations on handling and prevention of transmission. CoV are susceptible to heat, surfactants and low PH.

Read the study summary
Orofecal  Global - New Zealand
26 COVID-19: High temperature and high humidity reduce the transmission of COVID-19.

Some of the variations in COVID-19 transmission between cities worldwide can be explained by higher temperature and humidity being associated with lower transmission rates.

Read the study summary
Meteorological Urban environment  China and USA
27 COVID-19: Temperature significantly changes COVID-19 transmission in 429 cities

Lower weather temperatures were associated with higher COVID-19 transmission. There might be a most conducive temperature for the viral transmission, which may partly explain why it first broke out in Wuhan at that time.

Read the study summary
Meteorological  China
28 SARS-CoV-2 in Paris wastewaters

Quantitative monitoring of SARS-CoV-2 genomes in wastewaters may enhance surveillance intelligence of circulation at local or regional scales.

Read the study summary
Orofecal  France
29 SARS-Coronavirus-2 in sewage in the Netherlands

SARS-CoV-2 was detected in the sewage of five sites a week after the first COVID-19 case in the Netherlands. Even at low COVID-19 prevalence sewage surveillance could be a sensitive tool to monitor the viral circulation.

Read the study summary
Orofecal  Netherlands
30 SARS-CoV-2 Infection in Fecal Samples from Hong Kong

Fecal discharge continues long after respiratory shedding of COVID-19 has ceased.

Read the study summary
Orofecal  Hong Kong
31 COVID-19: Fecal-Oral Transmission of SARS-CoV-2 In Children

In children, the orofecal route is an alternative route of transmission, regardless of presenting COVID 19 symptomatology. Exclusion of SARS-CoV-2 infection by single time point nasopharyngeal swabs should not be used in children.

Read the study summary
Orofecal  Italy
32 Investigating SARS-CoV-2 surface and air contamination in a London hospital

Many hospital surfaces and air samples contained viral RNA. Viable virus was not cultured from any sample.

Read the study summary
Aerosol Fomites  UK
33 COVID-19: Evidence for gastrointestinal infection by SARS-CoV-2

SARS-CoV-2 may both infect the gastrointestinal system and transmit via a faecal-oral route. 

Read the study summary
Orofecal  China
34 COVID-19: Prolonged presence of SARS‐CoV‐2 viral RNA in faecal samples

In 98 hospitalized COVID-19 cases, patients’ faecal samples remained positive for SARS-CoV-2 for a mean of 11 days (maximum 5 weeks) after respiratory tract samples became negative.

Read the study summary
Orofecal  China
35 Fecal specimen diagnosis of novel coronavirus‐infected pneumonia

A small pilot sample of 14 cases  indicated agreement for the presence of COVID-19 between oropharyngeal sample and fecal samples.

Read the study summary
Orofecal  China
36 First detection of SARS-COV-2 In untreated wastewaters in Italy.

SARS-CoV-2 RNA fragments have been identified in sewage in Italy however the WHO protocol for sewage treatment of enveloped viruses after appropriate modifications is viable. The authors report a novel RT PCR test for screening of waters.

Read the study summary
Orofecal  Italy
37 COVID-19: Persistent viral shedding of SARS-CoV-2 in faeces

There is a high rate of PCR positive persistence of SARS-CoV-2 in faecal samples of patients with COVID-19.

Read the study summary
Orofecal  Global (Mainly China)
38 COVID-19 in wastewater

Recreational water could have concentration of SARS-CoV-2 agent via faulty wastewater management

Read the study summary
Orofecal  Global
39 COVID-19: Children with negative nasal specimens excrete SARS‐CoV‐2 in stools

Three children with mild symptoms who were SARS‐CoV‐2 throat swab specimen negative on discharge from hospital were stool positive 10 days post-discharge

Read the study summary
Orofecal  China
40 COVID-19: Viral loads in throat and anal swabs in children infected with SARS-CoV-2

In some children, fecal shedding may be a sign of prolonged mildly asymptomatic infection and represent the final phase of the disease.

Read the study summary
Orofecal  China, Wuhan
41 COVID-19: fecal shedding in children

Fecal shedding in children with COVID-19 occurs on average nine days longer than by the respiratory route

Read the study summary
Orofecal  Brazil/Global
42 COVID-19: Coronaviruses in water environments

Coroviridae have been isolated in different types of liquids from waste to surface water but in general, they appear to be unstable. Chlorination and higher temperatures lead to their inactivation.

Read the study summary
Orofecal Waterborne  Global
43 Epidemiological Characteristics of COVID-19 Close Contacts in Ningbo City

Transmission rates varied by closeness of contact and by type of contact. A single case travelling on a bus and attending a mass gathering appeared to be responsible for infecting 25 individuals

Read the study summary
Close contact Community Superspreading  China
44 Household secondary attack rate of COVID-19 and associated determinants.

COVID-19 cases in their presymptomatic period were at least as infectious as during their symptomatic period, and household contacts and elderly people were the most susceptible to infection.

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Close contact Person to person  China
45 Epidemiology and transmission of COVID-19 in Shenzhen, China

Case isolation and contact tracing reduces the time during which cases are infectious in the community.

Read the study summary
Close contact  China
46 Hospital transmission of COVID-19 in a general ward in Hong Kong

None of 120 contacts of a patient with initially undetected COVID-19 subsequently became infectious, suggesting SARS-CoV-2 is not spread by an airborne route. 

Read the study summary
Close contact Droplets  Hong Kong, China
47 COVID-19 Outbreak in a Call Center, South Korea.

High attack rates were identified for workers in an office space.  A map of the workspace and cases demonstrates the likelihood of close working contact increasing risk.

Read the study summary
Close contact Person to person  South Korea
48 Isolation of SARS-CoV-2 in untreated wastewater in Australia

SARS-CoV-2 was deleted for the first time in Australia using RT-qPCR assay, confirmed by sequencing

Read the study summary
Orofecal  Australia
49 Asymptomatic COVID-19 cases on board the Diamond Princess cruise ship

18% of cases were estimated to be asymptomatic on board the Diamond Princess cruise ship 

Read the study summary
Close contact Person to person  Japan
50 COVID-19: Estimates suggest COVID-19 transmission rates are highly seasonal

Seasonal temperature is associated with COVID-19 transmission globally, with 1°C increase in local temperature associated with 13% fewer cases.

Read the study summary
Meteorological  Global
51 The spread of SARs-CoV-2

Rapid spread may have been facilitated by significant numbers travelling outside of Hubei before lockdown.

Read the study summary
Person to person  China, Wuhan provinces other than Hubei
52 Potential faecal transmission of SARS-CoV-2

SARs-CoV-2 may be transmitted oro-fecally, but the number of studies in the present study with findings of viable virions in the faeces is small making this mode of transmission uncertain.

Read the study summary
Orofecal  China, Singapore, USA
53 COVID-19: Effects of temperature and humidity on the daily cases and deaths

Temperature and relative humidity were negatively correlated with daily new cases and daily new deaths of COVID-19.

Read the study summary
Person to person  166 countries (excluding China)
54 Factors influencing the sharp increase in COVID-19 in Northern Italy.

Administrative organisation issues, human bias, and bureaucracy in relevance to the epidemic are key factors in the extreme outbreak in Northern Italy.

Read the study summary
Meteorological Pollution  Italy
55 High transmissibility of COVID-19 near symptom onset.

The transmission was highest around the time of symptom onset and reduced quickly over subsequent days. Secondary infection was higher among household contacts than that in healthcare or other settings.

Read the study summary
Person to person  Taiwan, China
56 Transmission of COVID-19 to Health Care Personnel

Unprotected, prolonged patient contact, as well as certain exposures, including some aerosol-generating procedures, were associated with SARS-CoV-2 infection. 

Read the study summary
Nosocomial Person to person  California, USA
57 Transmission path of COVID-19 cluster cases in Zhuhai, China

Transmission in Zhuhai appears to be occurring mainly within close groups such as families.

Read the study summary
Person to person  China
58 Travel-Related Cases of SARS-CoV-2 in America.

No transmission among non-household contacts was detected. Among 15 household contacts, only 2 secondary cases (each the spouse of the respective case) were detected.

Read the study summary
Close contact Fomites Person to person  USA
59 SARS-CoV-2 infection in health care workers

Among 12 Health care workers diagnosed with COVID-19 asked to recall their working patterns pre-outbreak, poor sleep quality and high working pressure were positively associated with higher risks of COVID-19 diagnosis.

Read the study summary
Fomites Person to person  China
60 Transmission dynamics of children with COVID-19 in China

Children were more likely to have asymptomatic infections, milder conditions, faster recovery, and better prognosis than adults

Read the study summary
Person to person  China
61 Presymptomatic Transmission of SARS-CoV-2 in Singapore

Investigation of 243 cases of COVID-19 in Singapore identified seven clusters in which presymptomatic transmission was the most likely explanation for secondary infections.

Read the study summary
Presymptomatic transmission  Singapore
62 Transmission of the novel coronavirus onboard the Diamond Princess

Passengers and crew in the cruise ship experienced high transmission rates, hypothesised to be due to the close quarters and confined living spaces.

Read the study summary
Close contact Person to person  Japan
63 Close contact and risk of SARs-CoV-2 infection

In transmission events, after sharing a meal or staying at a chalet the secondary attack rate is estimated as very high.

Read the study summary
Close contact Person to person  China and France
64 COVID-19: Transmission Dynamics in Taiwan

Most transmission of COVID-19 occurred at the early stage or even before symptom onset, as symptoms developed and progressed the secondary attack rate decreased

Read the study summary
Person to person  Taiwan, China
65 A systematic review of SARS-CoV-2 transmission

The mean household SAR was 15% (95% CI: 12% to 19%) – ranging from 6.6% in Taiwan to more than 30% in four Chinese cities. Secondary attack rate from symptomatic index cases contact was greater than asymptomatics

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Close contact Person to person  Various
66 Atmospheric circulation and the spread of COVID-19 in Europe

Strong atmospheric stability with dry conditions favoured viral spread by short-range droplet transmission.

Read the study summary
Droplets Meteorological  Italy, Spain
67 Absolute humidity and transmission of COVID-19

Changes in weather alone will not necessarily lead to declines in COVID-19 case counts without the implementation of extensive public health interventions.

Read the study summary
Meteorological  SE Asia
68 COVID-19: Temperature, humidity and latitude analysis to predict potential spread and seasonality

The transmission of COVID-19 is associated with the 30 to 50 degree N’ longitude corridor and weather patterns and low specific and absolute humidity.

Read the study summary
Meteorological  Global
69 Effect of Temperature, humidity, and wind speed on COVID-19

Calm, cold, dry and overcast conditions are favourable to the transmission of COVID-19.

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Meteorological  310 regions from 116 countries by 12 March 2020
70 COVID-19: Transmission aboard the Diamond Princess cruise ship

Swift action by diagnosis, testing, contact-tracing and isolation would have probably avoided the majority of 619 cases aboard. 

Read the study summary
Close contact Fomites Person to person  International waters and Yokohama, Japan
71 COVID-19: Transmission amongst cruise ship passengers

Eighty-one percent of COVID 19 patients on a cruise ship were asymptomatic. Prevalence on affected cruise ships is likely to be underestimated and monitoring after disembarkation is necessary.

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Close contact Fomites Person to person  South Atlantic
72 COVID-19: Mechanisms for Accelerated Diffusion in Italy

The accelerated and vast diffusion of COVID-19 in Northern Italy was associated with the duration of cities’ exposure to polluted air.

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Pollution  Italy
73 Suppression of the COVID-19 outbreak in the municipality of Vò, Italy

In one of the two original epicentres of the outbreak in Northern Italy, Vò Euganeo, cases were  infected in the community before lockdown, or subsequently from asymptomatic infected people living in the same household.

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Person to person  Italy
74 Hospital outbreak of COVID-19 in South Africa

Separate access and segregation of potentially infected patients are required to prevent or control nosocomial outbreaks of COVID-19.

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Fomites Person to person  South Africa
75 COVID-19: Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia

COVID-19 transmission was occurring since mid-December 2019 in Wuhan: the majority in the Huanan Seafood Wholesale Market. A lack of early testing may have missed cases, particularly those with atypical symptoms.

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Person to person  China
76 Seasonal coronaviruses and establishing the context for COVID-19 emergence

Seasonal coronaviruses were detected in 4% of patients with respiratory illness who were tested at primary/secondary healthcare settings.

Read the study summary
Seasonality  Scotland, UK
77 SARS-CoV-2 Survival in Relation to Temperature and Humidity and Potential for Seasonality

Evidence suggests reduced transmission in high ambient temperatures and humidity, but it is limited and inconsistent.

Read the study summary
Meteorological  Worldwide
78 COVID-19: Early transmission dynamics in Nigeria.

The transmission was slow in the early weeks of the pandemic in Nigeria compared with other countries, which may be related to differences in case ascertainment.

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Community Imported  Nigeria
79 COVID-19: Climate and early global patterns of the outbreak

Temperature and humidity are strongly associated with the variation of the growth rate of Covid-19 cases across the globe in the early phase of the outbreak.

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Meteorological  189 countries/regions
80 COVID-19: weather, demographics and infection timeline.

In a multivariable model only population density and the timeline remained statistically significant.

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Meteorological  Worldwide
81 Association of Temperature and Humidity with Transmission of COVID-19 in China

Increases in temperature and humidity were associated with lower levels of  COVID-19.

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Meteorological  China
82 COVID-19: Associations of climate and public health interventions

Transmission of COVID-19 was associated with weakly associated with relative or absolute humidity and not associated with latitude and temperature.

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Meteorological  144 geopolitical areas worldwide
83 COVID-19: Presymptomatic Transmission in a Skilled Nursing Facility

On the basis of the reconstruction of an outbreak of COVID-19 in a skilled nursing facility, the presence of symptoms is not a sufficient indicator of infection. Viral shedding can continue for at least a week after testing positive.

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Person to person  USA
84 Infectivity of Asymptomatic SARS-CoV-2 Carriers

None of the 455 contacts of a single case of asymptomatic SARs-CoV 2 turned positive after a month.

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Close contact Droplets  Guangdong province, China
85 Transmission of COVID-19 in South Korea.

The transmission of SARS-CoV-2 in South  Korea was exacerbated by super spreading events in confined settings, including a hospital, a church and a gym.

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Person to person Superspreading  South Korea
86 Settings linked to SARS-CoV-2 transmission clusters

The results found evidence of SARS-CoV-2 transmission clusters for 152 events, which was classified into 18 types of settings.

For the references see the online database.

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Close contact Person to person  Worldwide (results mostly from Asia)
87 Aerosol Transmission of COVID-19

Transmission of COVID-19 may take place by fine droplets emitted by infected people with no symptoms when they speak or breathe without necessarily coughing or sneezing.

Read the study summary
Droplets  UK
Funding

This work is supported by the National Institute of Health Research Evidence Synthesis Working group, and by Maria and David Willetts.