COVID Odyssey: Winter Warning NZ ~ Delta variant ~ May one person infect 12 others?

Based on NZ current case numbers for the Delta variant, we estimate on average one person may infect at least 12 others (Ro > 12) without quarantine or isolation. Case numbers may close to triple over a two-day period (1.7 x 1.7 = 2.89).

For the Delta variant worldwide, we have estimated Ro to be in the range 9 to 12.7.

Ro is the number of people on average one person may infect without quarantine or isolation. For more definitions se below.

We estimate the daily increase, r = 1.7, for the Delta variant.

Using simulations, we have developed a formula to estimate Re and Ro using r and daily case numbers.

Using our formula, for r = SQRT(3), Ro = 12.7 (1 dp), we may have case numbers tripling every two days (for the Delta variant).

Note that NZ case numbers only reached 30 on Friday. Hence our estimate is a very preliminary estimate. We calculate r = 51/30 and use our formula for Ro below to estimate Re.

We obtain using our formula, Re is close to 12 and hence Ro is likely to be greater than 12.

We calculate Ro and Re from r using our formula:

We assume n = 10 days and that infectivity decreases daily at a rate of 1/r.

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 also consider:

COVID Odyssey: Winter Windup 13 ~ Is the COVID-19 Delta variant as infectious as measles?

Experts think the COVID-19 Delta variant is much more contagious than the flu and chickenpox, and on par with the measles. See:
https://www.healthline.com/health-news/cdc-says-delta-variant-as-infectious-as-chickenpox-what-to-know-now

For these values of Ro and herd immunity thresholds (HITs) of well-known infectious diseases prior to intervention, see:
https://en.wikipedia.org/wiki/Basic_reproduction_number

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 have chosen n = 10 days. In the tables below, if n > 10 days, the values for Ro are likely to be too high to be acceptable.

We have also used this version:

You may like to look at:

COVID Odyssey: Winter Windup 15 ~ Executive summary ~ Our formula for Ro

https://aaamazingphoenix.wordpress.com/?s=windup

I believe that New Zealand has done the right thing moving to Lockdown Level 4 quickly this week at 11.59pm on Tuesday 17 August  when a case of the Delta variant was discovered in the community.

Let’s hope New Zealand does not need to stay in Lockdown Level 4 for a lengthy period.

Alan Grace
22 August 2021

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