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​​​​​​​Epidemiology : Background & Cases


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Epidemiology

Background
On 31 December 2019, the World Health Organization (WHO) detected reports of a cluster of viral pneumonia cases of unknown cause in Wuhan, Hubei, and an investigation was launched at the start of January 2020. On 30 January, the WHO declared the outbreak a Public Health Emergency of International Concern (PHEIC)—7,818 cases confirmed globally, affecting 19 countries in five WHO regions.

Several early infected people had visited Huanan Seafood Wholesale Market; the virus is therefore thought to be of zoonotic origin. The virus that caused the outbreak is known as SARS‑CoV‑2, a newly discovered virus closely related to bat coronaviruses, pangolin coronaviruses, and SARS-CoV. The scientific consensus is that COVID-19 has a natural origin. The probable bat-to-human infection may have been among people processing bat carcasses and guano in the production of traditional Chinese medicines.

The earliest known person with symptoms was later discovered to have fallen ill on 1 December 2019, and that person did not have visible connections with the later wet market cluster. Of the early cluster of cases reported that month, two-thirds were found to have a link with the market. On 13 March 2020, an unverified report from the South China Morning Post suggested a case traced back to 17 November 2019 (a 55-year-old from Hubei) may have been the first person infected.

The WHO recognised the spread of COVID-19 as a pandemic on 11 March 2020 as Italy, Iran, South Korea, and Japan reported surging numbers of cases. The number of cases outside China quickly surpassed the number of cases inside China.

Cases
Official case counts refer to the number of people who have been tested for COVID-19 and whose test has been confirmed positive according to official protocols. As of 24 May, countries that publicised their testing data have typically performed many tests equal to 2.6 percent of their population, while no country has tested samples equal to more than 17.3 percent of its population. Many countries, early on, had official policies to not test those with only mild symptoms. An analysis of the early phase of the outbreak up to 23 January estimated 86 percent of COVID-19 infections had not been detected, and that these undocumented infections were the source for 79 percent of documented cases. Several other studies, using a variety of methods, have estimated that numbers of infections in many countries are likely to be considerably greater than the reported cases.

On 9 April 2020, preliminary results found that 15 percent of people tested in Gangelt, the centre of a major infection cluster in Germany, tested positive for antibodies. Screening for COVID-19 in pregnant women in New York City, and blood donors in the Netherlands, has also found rates of positive antibody tests that may indicate more infections than reported. Seroprevalence based estimates are conservative as some studies shown that persons with mild symptoms do not have detectable antibodies. Some results (such as the Gangelt study) have received substantial press coverage without first passing through peer review.

Analysis by age in China indicates that a relatively low proportion of cases occur in individuals under 20. It is not clear whether this is because young people are less likely to be infected, or less likely to develop serious symptoms and seek medical attention and be tested. A retrospective cohort study in China found that children were as likely to be infected as adults. Countries that test more, relative to the number of deaths, have a younger age distribution of cases, relative to the wider population.

Initial estimates of the basic reproduction number (R0) for COVID-19 in January were between 1.4 and 2.5, but a subsequent analysis concluded that it may be about 5.7 (with a 95 percent confidence interval of 3.8 to 8.9). R0 can vary across populations and is not to be confused with the effective reproduction number (commonly just called R), which takes into account effects such as social distancing and herd immunity. By mid-May 2020, the effective R was close to or below 1.0 in many countries, meaning the spread of the disease in these areas at that time was stable or decreasing.

wikipedia.org

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