COVID - Epidemiology

By Dr Deepu Changappa Cheriamane

As of July 2020, the number of cases of confirmed COVID-19 globally is over 11 million affecting virtually every territory, other than isolated South Pacific island states and Antarctica, according to an online virus tracker created by the medical journal, The Lancet, and hosted by Johns Hopkins University. As of June 2020, the United States had more than two million cases, Brazil more than one million, with Russia and India with >500,000 cases. 

The R0 (basic reproduction number) of SARS-CoV-2 has been estimated between 2.2 and 3.28 in a non-lockdown population, that is each infected individual, on average, causes between 2-3 new infections. 
The incubation period for COVID-19 was initially calculated to be about five days, which was based on 10 patients only. An American group performed an epidemiological analysis of 181 cases, for which days of exposure and symptom onset could be estimated accurately. They calculated a median incubation period of 5.1 days, that 97.5% became symptomatic within 11.5 days (CI 8.2 to 15.6 days) of being infected, and that extending the cohort to the 99th percentile results in almost all cases developing symptoms in 14 days after exposure to SARS-CoV-2.
As of June 2020 the number of deaths from COVID-19 passed half a million globally. The case fatality rate is ~2-3%. It is speculated that the true case fatality rate is lower than this because many mild/asymptomatic cases are not being tested, which thus skews the apparent death rate upwards.
A paper published by the Chinese Center for Disease Control and Prevention (CCDC) analyzed all 44,672 cases diagnosed up to 11 February 2020. Of these, ~1% were asymptomatic, and ~80% were classed as "mild". 
Another study looked at clinical characteristics in COVID-19 positively tested close contacts of COVID-19 patients. Approximately 30% of those COVID-19 positive close contacts never developed any symptoms or changes on chest CT scans. The remainder showed changes in CT, but ~20% reportedly developed symptoms during their hospital course, none of them developed severe disease. This suggests that a high percentage of COVID-19 carriers are asymptomatic.
In the Chinese population, 55-60%% of COVID-19 patients were male; the median age has been reported between 47 and 59 years.
Children seem to be relatively unaffected by this virus, or indeed other closely-related coronaviruses.with large cohort studies reporting that 1-2% of COVID-19 patients are children. However, there have been cases of critically-ill children with infants under 12 months likely to be more seriously affected. A very low number of pediatric deaths has been reported . In children, male gender does not seem to be a risk factor. The incubation period has been reported to be shorter than in adults, at about two days.

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