Why we corrected our estimates for the reproduction number of two COVID subvariants

We published an article earlier this month that contained incorrect estimates for the basic reproduction number (R0) for COVID subvariants BA.4/BA.5 and BA.2. We have now corrected the error, but I wanted to take a minute to explain what went wrong and how we are addressing it.

The article – Australia is heading for its third Omicron wave. Here’s what to expect from BA.4 and BA.5^[1] – estimated that the basic reproduction number (or R0) of Omicron subvariant BA.2 was about 13.3 and BA.4/5 was 18.6, which is similar to measles.

As a number of scientists, as well as a Reuters fact check^[2], have now pointed out, these estimates are likely to be incorrect. That’s because R0 only takes into account “intrinsic transmissibility”, whereas a key factor contributing to the growth of BA.4/5 is “immune escape” – the ability of the subvariant to evade immunity from vaccination or previous infection.

We have corrected the BA.4/5 article^[3]. And because we have used this simple but incorrect calculation previously on The Conversation (that is, a previous variant’s R0 multiplied by how much more infectious a new subvariant is), we will go through other affected articles and correct where necessary.

The epidemiological concepts that lie behind this error are tricky to explain, so we’ve also commissioned a fresh article by experts in the field, explaining what R0 is (and isn’t).

The Conversation exists to provide reliable information and when we identify errors we aim to correct them and be as transparent as possible about what went wrong and what we are doing about it.

Republishers of the article have been notified of the correction. Going forward, we won’t use simple calculations to estimate the R0.

References

1. ^{^} Australia is heading for its third Omicron wave. Here’s what to expect from BA.4 and BA.5 (theconversation.com)
2. ^{^} Reuters fact check (www.reuters.com)
3. ^{^} corrected the BA.4/5 article (theconversation.com)