Epidemiology

This page collects observations, interpretations, and consequences for action about the epidemiology of SARS-CoV2 in general. Please observe the structure of the page, when you add your content. Please use references where possible. Remember to find the relevant page. For example, if your observation is about transmission routes, please use that page, instead of posting your content here.

What is already known

• Coronavirus disease (COVID-19) is caused by SARS-COV2 and is a potentially fatal disease that is designated a PHEIC by WHO .

Symptoms

• The predominant symptoms in laboratory-confirmed cases is fever (88%) and dry cough (68%), followed by fatigue (38%), sputum production (33%), dyspnoea (19%), sore throat (14%), headache (14%) and myalgia or arthralgia (15%). Less common symptoms are diarrhoea (4%) and vomiting (5%).
• About 80% of reported cases in China had mild to moderate disease (including non-pneumonia and pneumonia cases), 13.8% had severe disease and 6.1% were critical (respiratory failure, septic shock, and/or multiple organ dysfunction/failure). Current estimates suggest a median incubation period from five to six days for COVID-19, with a range from one to up to 14 days.
• Italy reports consistently between 9% and 11% of the symptomatic cases to require intensive care treatment.

Case fatality

• Reliable estimates for case fatality for COVID-19 are still lacking and biased. Based on a large dataset from cases in China, the overall case-fatality risk (CFR) among laboratory-confirmed cases was higher in the early stages of the outbreak (17.3% for cases with symptom onset from 1-10 January) and has reduced over time to 0.7% for patients with symptom onset after 1 February. In data on diagnosed COVID-19 cases in China, Italy and South Korea, overall CFR was 2.3%, 2.8% and 0.5%, respectively, and increased with age in all settings, with the highest CFR among people over 80 years of age (14.8%, 8.2% and 3.7%, respectively).

Viral shedding

• Over the course of the infection, the virus has been identified in respiratory tract specimens 1-2 days before the onset of symptoms and it can persist for 7-12 days in moderate cases and up to 2 weeks in severe cases.
• In faeces, viral RNA has been detected from day 5 after onset and up to 4 to 5 weeks in moderate cases.
• SARS-CoV2 has been detected also in whole blood, serum, saliva and urine.
• Prolonged viral RNA shedding has been reported from nasopharyngeal swabs, up to 37 days among adult patients and in faeces, for more than one month after infection in paediatric patients.

Basic reproduction number (R0)

• The current estimates of the basic reproductive number R0 are between 2 and 3 in settings from China and during the early stage of an outbreak on a cruise ship.

Asymptomatic Infections

• Based on Chinese data, the international WHO mission report indicates that up to 75% of initially asymptomatic cases will progress to clinical disease, making the true asymptomatic infection rather rare (estimated at 1-3%).
  • The virus has been detected in asymptomatic persons. On a rapidly evolving cruise ship outbreak, where most of the passengers and staff were tested irrespective of symptoms, 51% of the laboratory-confirmed cases were asymptomatic at the time of confirmation. In Italy, 44% of the laboratory-confirmed cases have been asymptomatic. In Japan, 0.06% of reported cases have been asymptomatic. These proportions based on nationally notified cases likely reflect laboratory testing algorithms rather than true estimates of asymptomatic infections.
  • Both viral RNA and infectious virus particles were detected in throat swabs from two German citizens evacuated from Hubei province on 1 February 2020, who remained well and afebrile seven days after admission to a hospital in Frankfurt. Both a mother and a child in a family cluster remained asymptomatic (including normal chest CT images during the observation period) with qRT-PCR positive nasopharyngeal swab samples. Similar viral load in asymptomatic versus symptomatic cases was reported in a study including 18 patients. Persistent positivity of viral RNA in throat and anal swabs were reported in an asymptomatic female patient after 17 days of clinical observation and treatment.
• Potential transmission from an asymptomatic person has been reported in a familial cluster of five COVID-19 patients hospitalised with fever and respiratory symptoms that had contact before their onset of symptoms with an asymptomatic family member, a young 20-year-old woman, upon her return from Wuhan. She remained asymptomatic for the whole duration of laboratory and clinical monitoring (19 days).
• Transmission in a pre-symptomatic stage of infection: In addition to case reports, the pre-symptomatic transmission has been inferred through modelling, and the proportion of pre-symptomatic transmission was estimated to be around 48% and 62%. The pre-symptomatic transmission was deemed likely based on a shorter serial interval of COVID-19 (4.0 to 4.6 days) than the mean incubation period (five days) with the authors indicating that many secondary transmissions would have already occurred at the time when symptomatic cases are detected and isolated. Major uncertainties remain in assessing the influence of pre-symptomatic transmission on the overall transmission dynamics of the pandemic.

Vulnerable groups

• Population groups that have been more frequently reported having severe disease and death include people above 60 years of age, males, people with underlying conditions such as hypertension, diabetes, cardiovascular disease, chronic respiratory disease and cancer.
• The proportion of most of the reported chronic diseases and health conditions is similar to the prevalence of these conditions in the elderly age groups in China, therefore they might be surrogates of increasing age only. Higher ACE (angiotensin-converting enzyme II) gene expression may be linked to higher susceptibility to SARS-CoV-2.
• It has been shown that ACE 2 expression in lung tissues increases with age, tobacco use and with some hypertensive treatment. These observations might explain the vulnerability of older people, tobacco users/smokers and those with hypertension; they also highlight the importance of identifying smokers as a potential vulnerable group for COVID-19.
• There is limited scientific evidence on the severity of illness among pregnant women with COVID-19. Pregnant women appear to experience similar clinical manifestations as non-pregnant adult patients with COVID-19 pneumonia. There is no evidence of severe adverse outcomes in neonates due to maternal COVID-19 pneumonia, and the virus has not been found in breastmilk. .
• A recent retrospective review of medical records by Chen et al. identified two cases of fetal distress among the nine monitored pregnancies, but did not identify any evidence of intrauterine infection.
• Currently available information indicates that children are as likely to be infected as adults, however they experience mild clinical manifestations. About 2.4% of the total reported cases in China (as of 20 February 2020) were individuals under 19 years of age. A very small proportion of those aged under 19 years have developed severe (2.5%) or critical disease (0.2%).

Children

• From early on, it was clear COVID19 attack rates in children were low. In Wuhan, no children tested positive between November 2019 and mid-January, per an early WHO report.
• A study of 391 cases and 1,286 close contacts in Shenzhen, China, showed that kids under 9 had an attack rate of 7.4%, similar to adults (though adults 60-69 had a higher attack rate of 15.4%).
• Overall, children were at a similar risk of infection as the adult population, though less likely to have severe symptoms, if infected.
• Another sophisticated study using multiple sources of data concluded that, compared to those aged 30–59 years, those aged <30 years and >59 years are 0.16 and 2.0 times more susceptible to symptomatic infection.
• Transmission from pregnant women to their children in utero (known as ‘vertical transmission’) seems rare for COVID-19, too. This is similar to SARS (from 2003).
• Not only is the symptomatic attack rate (mind you: not the Infection Rate) low in children (which we will need to confirm, eventually, with 'serological' tests in surveys), but the fatality rate, among those kids who do get infected, is VERY low.

Reservoir and sources

• The three first reported cases were hospitalized on 27 December 2019 in Wuhan; one patient was a retailer on the seafood market in Wuhan, another was a frequent visitor of the same market. The third patients was not reported to be associated to the market.
• COVID19 is likely of animal origin. The virus has 85% similarity to a strain that was isolated from bats in East China.

Transmission Routes

• Person-to-person transmission of COVID-19 infection is the predominant way in which this virus spreads among the human population. . Transmission occurs through close contact (droplets, hands) and fomites. Therefore handwashing and social distancing are effective control measures, in the absence of a vaccine.
• Extensive measures to reduce person-to-person transmission of COVID-19 have been implemented to control the current outbreak.
• Special attention and efforts to protect or reduce transmission should be applied in susceptible populations including children, health care providers, and elderly people.

Detailed pages

For the following specific epidemiological topics, please use the subpages to document your observations:

• • Reservoirs
    • Sources
  • Transmission Routes
  • Transmission Barriers
  • Immune Response
  • Pathogenesis
    • Risk factors for severe disease
  • Contagiousness

For other epidemiological topics, please use this page.

Proportion asymptomatic

Observations

• A study of the COVID19 outbreak on the Pacific Princess cruise ship found proportions of asymptomatic infections up to 50%
• Based on Chinese data, the international WHO mission report indicates that up to 75% of initially asymptomatic cases will progress to clinical disease, making the true asymptomatic infection rather rare (estimated at 1-3%).

Analysis and interpretation

• The observations by Mizumoto and the International WHO Mission seem to contradict on the issue of proportion asymptomatic infections. Though cruise ship populations are hardly representative of national populations, they can be considered reliable environments to study the natural history of infectious disease, when observing large groups of exposed susceptibles.
• Having reliable estimates of the proportion asymptomatics is relevant for modelling the disease at the population level and making predictions about group immunity and transmission coefficients.

Consequences for action

• Ideally, cohorts of exposed individuals need to be identified and followed up to establish the proportion asymptomatic more reliably.

Anosmia

Observations

• An increasing number of reports occur on the sudden loss of smell and taste among a proportion of the COVID19 patients.

Analysis and Interpretation

• This observation requires a systematic clinical follow up of COVID19 patients, including pathognomonic studies

Consequences for Action

• If the relation COVID19 and Anosmia is confirmed, then clusters of sudden anosmia could be considered a clinical indicator of COVID19 in the population when testing is unavailable.