I took a look at Mumbai's #COVID19 data by age and came to some worrying conclusions. I'd encourage people celebrating seemingly low IFR values to look more closely. I used: Spain's IFR data; and Mumbai's age pyramid, age-structured fatality data, and seroprevalence data. 1/6

Naive IFR from Mumbai's seroprevalence and fatality data is ~0.12% (not, I think, 0.05-0.1% as is widely quoted).

An "expected IFR" for Mumbai, using Mumbai's age pyramid and Spain's age-dependent IFR is ~0.22%.

These two values hide a more complicated story. 2/6

IFR in some age groups, particularly 40-60, appears to be quite a lot higher (about 60% higher) in Mumbai than in Spain, even *assuming no death undercounting*. If there is death undercounting in this age group, then IFR goes up further. This should be a cause for concern. 3/6

The apparently low overall IFR from seroprevalence data is almost entirely a consequence of lower than expected IFR for the over-70s. This cancels out the higher than expected IFR amongst 40-60 year olds. We have to ask: are the elderly really doing so well? 4/6

Either the elderly in Mumbai have some natural, as yet unclear advantage over the elderly in Spain *which is not shared by younger age groups in Mumbai*, or fatality undercounting is particularly high amongst the over 70s. (The latter is my guess.) 5/6

Full write-up here. References to data sources in this document. 6/6
http://maths.mdx.ac.uk/research/modelling-the-covid-19-pandemic/a-quick-look-at-mumbais-seroprevalence-and-fatality-data/