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SA's latest Covid-19 reproductive number falls to just above 1

The National Institute for Communicable Diseases.says SA's Covid-19 reproductive number is down to 1.1..
The National Institute for Communicable Diseases.says SA's Covid-19 reproductive number is down to 1.1..
Image: 123rf.com/betonstudio

SA's Covid-19 reproductive number is down to 1.1, says the National Institute for Communicable Diseases.

The number — commonly known simply as R — is the average number of secondary infections produced by a typical case of an infection in a population where everyone is susceptible.

SA's average R during the alert level 5 lockdown that began at the end of March was 1.33, though it was as high as 1.5 by the end of April.

During the level 4 lockdown it dropped to an average of 1.26, and between June 1 and July 5, when SA was in level 3 lockdown, it averaged 1.1.

The red line shows the decline of the Covid-19 reproductive number (R) since SA's first mortality on March 27. The shaded area represents a 95% confidence interval.
The red line shows the decline of the Covid-19 reproductive number (R) since SA's first mortality on March 27. The shaded area represents a 95% confidence interval.
Image: National Institute for Communicable Diseases

“This indicates ongoing transmission at a steadily slowing rate over this period,” said the NICD in its first report about SA's R since June 12.

The Western Cape R, which averaged 1.45 during level 5 lockdown when infections in the province raced ahead of the rest of the country, is now the lowest, averaging exactly 1 between June 1 and July 5. When R moves below 1, the number of cases is declining.

In Gauteng and the Eastern Cape, the R fell steadily during level 3 lockdown, but the NICD said it remained well above 1 in KwaZulu-Natal, “indicating increasing infections in this province”. R is not available for other provinces.

The NICD said it had changed its method of calculating R since its previous report, which used data on laboratory-confirmed Covid-19 cases. The new estimates are based on lab-confirmed Covid-19 deaths.

“The reason for the shift in estimation approach is the changes in laboratory testing practices since June, leading to fluctuations in numbers of tested and confirmed cases which may not accurately reflect the epidemic trajectory,” it said.

“It is felt that laboratory-confirmed Covid-19 deaths are likely a more stable indicator of Covid-19 epidemic progression, albeit with a substantial time lag, which is a disadvantage of this approach.”

However, the new R estimate used data available until July 26 which were adjusted for the delay between the onset of illness and death, then “right-censored” backwards for 21 days to account for reporting delays.

Another possible problem with the new method, the NICD said, was the introduction of dexamethasone treatment and high-flow nasal oxygen in mid-June, both of which may have lowered mortality.

“It is important to interpret these findings together with other sources of data on transmission,” it said.

By Friday evening, SA had 603,338 confirmed cases of Covid-19, with 12,843 deaths (2.1%), and 500,102 recoveries (81.9%). The remaining 90,393 cases (15%) are still active.

The number of active cases is now just above half a high of 173,590 reported on July 20, and is falling by 24% every fortnight, based on a 14-day moving average.

TimesLIVE

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