COVID Odyssey: Winter warning ~ Summary 2020 revisited ~ Daily cases and how many people one person may infect on average?

In 2020, We found that in New Zealand given a a 10-day infectious period, and a daily increase in COVID-19 infections of r = 1.4, one person with COVID-19 may infect on average 4.14 other people and for a 15-day infectious period, one infected  person may infect around 6 other people.

We extend our previous results to show values for Ro (or Re) between 3.5 and 7.5.

Update: See:
Search Results for “windup” – COVID Odyssey by Alan Grace: My vir[tu]al COVID-19 journey (wordpress.com)

Since early calculations (e.g. by WHO) assumed a value for Ro of between 1.4 and 2.5, our results are a significant increase over the original estimates and most current Ro values. See:
https://www.who.int/news/item/23-01-2020-statement-on-the-meeting-of-the-international-health-regulations-(2005)-emergency-committee-regarding-the-outbreak-of-novel-coronavirus-(2019-ncov)

In this post we consider again the formula developed last year during simulations for the spread of COVID-19 in New Zealand to calculate (Ro) the number of people one person with COVID-19 may infect on average during the initial outbreak in early 2020 (without quarantine or self-isolation) and consider Re, the effective reproduction rate with quarantine and self-isolation.

We assume a homogeneous population where the spread within the population is similar throughout the population.

New Zealand is relatively isolated and therefore is suitable for estimating the spread of COVID-19 in general in the world.

By definition we let:

• r denote the effective Reproduction rate of COVID-19 for one day
• Ro (R0; R-Zero; R-Nought) denote the Reproduction number for COVID-19 without any quarantine or isolation
• Re denote the effective Reproduction number for COVID-19
  (Re assumes isolation/quarantine is happening)
• a case be defined as a person diagnosed as having COVID-19

Note that Ro and Re are numbers (not rates), the number of people one person with COVID-19 may infect on average without quarantine or isolation (Ro) and with quarantine or isolation (Re).

We assume a person may be infectious for n days once they become symptomatic (show symptoms). A person may be infectious for at least two days prior to becoming symptomatic.

We assume a 5-day cycle where the incubation period is one cycle long (until symptoms appear) and the infectious period is two cycles long.

In New Zealand we have found a daily rate of increase of r ~ 1.4 (40% increase per day) in March and assume a 10-day infectious period (when a case can infect others).

We assume a daily increase of r (e.g. r = 1.4) and a daily decay in infectivity of 1/r over a 10-day infectious period for each case. We also consider r = SQRT(2) [~1.4142]

Our findings produced the formula in the table below to calculate Ro, where n is the number of days a person is infectious. We show values for Ro (or Re) between 3.5 and 7.5.

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