COVID Odyssey: NZ New Year Fear 8 Post #935~ NZ Omicron: Estimating Ro

We use actual case numbers for the Omicron variant in New Zealand to estimate Ro.

We find that case numbers may potentially triple in a two-day period.

For update see:
https://aaamazingphoenix.wordpress.com/2022/02/01/covid-odyssey-nz-new-year-fear-11-post-938-nz-covid-19-can-ro-be-30-higher-than-re/

To estimate Ro (the average number of people one person may infect without isolation over a 10-day infectious period), we need to estimate (project) the number of people a case may infect if the case had not been isolated.

For background see:
https://aaamazingphoenix.wordpress.com/cover-photo/

Step 1

We look at the daily increase (r). We use this to project additional cases.

Below are actual case numbers (first two columns):

Note that on 26 January there are 12 historic cases that have not been included as cases for the day (they are in the total).

“In addition, there is a total of 12 cases linked to the three border workers reported earlier this month with confirmed or suspected Omicron, an increase of six.” See:
https://www.health.govt.nz/news-media/news-items/more-100000-first-doses-5-11s-90-auckland-maori-fully-vaccinated-23-community-cases-6-hospital

On 28 January we had only 15 new cases. The curve appears to be starting to flatten at least until we have some super-spreader events.

Step 2

We have found that a weighted average works better to estimate r.

We see that r is close to SQRT(3).

This means that case numbers could triple over a two-day period.

Compare the second Total column with the last column.

Step 3

We use the weighted average column for the factor.

Step 4

We use 0.732 as the Factor.

We see on 27 January the totals are close.

We look at this table:

We see that for r = 1.732, Ro ~ 12.73.

We also see that for r = 1.66 (Step 2 and Step 3), Ro ~ 11.

1.73 – 1.66 = 0.07

1.73 + 0.07 = 1.8

For r = 1.8, Ro ~ 14.4

These estimates (Ro from 11 to 14.4) are within our previously estimated range for Omicron. See:
https://aaamazingphoenix.wordpress.com/2021/12/17/covid-odyssey-summer-surprise-2-post924-nz-covid-19-omicron-has-reached-nz-border/

https://www.health.govt.nz/news-media/news-items

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