The data you link would not support your conclusion. It’s not flu season in UK, so the constant low rate is within normal variation, even if mask-wearing is impacting decreased flu transmission. You’re talking about ~100 deaths for each covid and flu in the time period cited, and for covid (which so far doesn’t have a cycle like flu) the effect on decreasing is very stark.
In other words, this particular data would have nothing to say. It’s consistent with random variation of off-season flu, it’s also consistent with masks having a big effect on reducing covid spread (especially with covid’s much higher transmission rate and airborne properties), and it’s also consistent with some third exogenous factor causing the covid reduction in a way that has no bearing on flu.
If one of two airborne transmissions remains within normal variation while the other drops significantly, then masks had an insignificant effect.
The more plausible explanation is that mask wearing had a tiny impact on both covid and flu. The reduction in covid results from something else. Not everything has to do with masks. Masks don't discriminate between flu and covid.
> “ If one of two airborne transmissions remains within normal variation while the other drops significantly, then masks had an insignificant effect.”
Statistically speaking, it’s a large error to assert this or draw this conclusion.
There are too many confounders, the hugest being that it’s not flu season there and the sample size is way too low to have confidence in any conclusion based on the UK flu sample. Given the significance of the data in southern hemisphere countries, it suggests there is an effect on flu transmission and the UK data (by confounders and small sample) are consistent with that possibility.
> “Masks don't discriminate between flu and covid.”
That’s right - suggesting a big effect on reducing both, with confounders that would make such a comparison meaningless if you tried to make it in a country like UK right now.
In other words, this particular data would have nothing to say. It’s consistent with random variation of off-season flu, it’s also consistent with masks having a big effect on reducing covid spread (especially with covid’s much higher transmission rate and airborne properties), and it’s also consistent with some third exogenous factor causing the covid reduction in a way that has no bearing on flu.