web analytics

To mask or not to mask – my take on the recent Cochrane systematic review

Masking during the Covid pandemic has always attracted very polarised views. My take on it in March 2020 was to be sceptical:

You can see a lot of people wearing them, but the efficiency of the masks is actually minimal. 


The point I was making in March 2020 was that badly worn masks are actually counterproductive. I changed the view a bit later in June 2020:

Hence, any face covering that blocks the droplets will do the job.


but still wrote in July 2021:

Simple masks are effective, but mostly for protecting the public from the person who wears them, not the other way round.



 I do not like wearing a mask, although I have invested in a high-quality one by Airinum having worn similarly excellent homemade masks at the start of the epidemic.


In the 2018/19 flu pandemic, people were encouraged to contain the droplets associated with coughing and sneezing:

Actually, even before that, The Good Soldier Schweik would use every opportunity to spit into special vessels provided to control TB:

Cochrane review

So, how does the Cochrane review actually say? The authors are very cautious (as they should be). They conclude as follows:

Surgical masks

Wearing medical surgical masks probably makes little or no difference to the outcome of influenza-like illness (ILI)/COVID-19 like illness compared to not wearing masks.

The risk ratio (RR) is 0.95. What does this mean? It means that your risk of catching the disease (flu or Covid) if you wear a mask is slightly lower than the risk associated with not wearing.

But, the 95% confidence interval (CI) is 0.84 to 1.09, so we are 95% certain that the risk ratio is anything between either pretty good (0.84) or a bit rubbish (1.09).

Actually, we can also say that there is a 2.5% chance that masks reduce the risk by 16% or more. Conversely, there is a 2.5% chance that masks are more rubbish.

Fancy masks

This is a mask I still occasionally wear (more precisely, it is K95-certified which is similar but not identical to the US N95 or EU P2). The conclusion for N95/P2 masks is:

We are very uncertain on the effects of N95/P2 respirators compared with medical/surgical masks on the outcome of clinical respiratory illness (RR 0.70, 95% CI 0.45 to 1.10; 3 trials, 7779 participants; very low-certainty evidence).


To unpack this “mumbo-jumbo” of numbers, we are bit more certain that N95/P2 masks work (the risk ratio is 0.7, so the reduction could be 30% but as before, we can only say that we are 95% certain of RR being between 0.45 – a reduction by 55% – and 1.1 – an increase by 10%.

Some thoughts on this figures

As a statistician, I am always a bit wary of interpreting results of poorly designed experiments with many confounding factors and small replication. I am even more wary of meta-analysis studies like the Cochrane review, as there is a lot of arbitrariness in selecting and interpreting results.

It is absolutely OK if the results are clear-cut, but the large confidence intervals make me very sceptical. I suspect that in the studies included here, it was almost impossible to sufficiently control all factors associated with mask wearing, to draw any firm conclusions.


So, my TL&DR is that there is simply not enough evidence gathered by this type of experiments to draw any firm conclusions. If anything, there is a slight preference for risk reduction, but we cannot make this statements for sure.

The authors themselves start their conclusions that:

The high risk of bias in the trials, variation in outcome measurement, and relatively low adherence with the interventions during the studies hampers drawing firm conclusions.


Which masks, when, how and by whom were worn, matters. As do the type of disease, its mode of transmission, and overall human behaviour.

And that’s before we start taking into account possible drawbacks of the methodology and arbitrariness of assigning weights, as Tomas explains in his thread.

So, does it all matter?

Yes, because the next time we are hit with another pandemic, we will be without evidence that one of the simplest ways to control spread is missing because people will not take them up.

The Cochrane authors conclude that:

There is a need for large, well-designed RCTs [Random Controlled Trials] addressing the effectiveness of many of these interventions in multiple settings and populations, as well as the impact of adherence on effectiveness, especially in those most at risk of ARIs [Acute Respiratory Infections].


I finish with a quote from my July 2021 blog post:

But I am really puzzled by such a strong resistance to the idea of mask-wearing. Even in Western countries, we are taught from childhood that we should be doing things that protect us – and others – from risk, even if [they come at] a cost. “Do to others what you would have them do to you” is the Golden Rule widely accepted in our culture. How is it then that so many people in the West feel so strongly against mask-wearing?


In the last couple of days, there has been a lot of discussion on Tomas Pueyo thread cited above, pointing out problems with his comments. My article did not rely on his analysis and so I left it unedited.

Major infectious diseases – England (and Scotland) update, week 7

I have now been away for almost a week, attending a conference in India – with very different temperatures to my home in Scotland – 33C instead of 6C.

TL&DR: Covid cases, hospitalisations, and deaths are sadly still going up. Flu and RSV cases are stabilising, so we might see some more weeks with substantial numbers. Scarlet fever numbers stay high but stable and get close to pre-pandemic levels.


Starting this time with reported cases, the numbers continue to climb up in all age classes except those under 14 years old. Maybe, just maybe, there is a hint of slowing down (particularly in Scotland), but we will need to wait for another week to see what happens next.

The current wave is driven by the XBB variant, now dominating in the US and building up in other countries, as seen here:


and here

Hospitalisations and deaths are also climbing up (the last few points on the deaths plot will almost certainly be revised up next week).


Influenza cases are slightly up, although I expect this will just be a short-term increase. The 2022/23 season is so similar to the 2019/20 season that I suspect we will see a long “tail” of relatively low-level flu wave.


As for flu, RSV seems to follow the pre-pandemic pattern, slowly declining towards the Spring and Summer lows. The return of high numbers as in Summer 2021 is, in my opinion, highly unlikely.

Scarlet fever

As expected, the number of cases is now stabilising (and maybe even slightly increasing). I originally thought there would be more increase – following the pre-pandemic trends rather than absolute values, but it seems I was wrong.

I now expect the numbers to be stable and similar to the “bumper” 2018 March and April values.


I am not sure what is going on here – the number of reports is very high, almost double the 5 season average and well above 97.5% mark (upper value of the 95% interval).

In the report, this is attributed to increased reporting, and compared to outbreaks over the last decade (but only the last 5 years shown).

I suspect it is indeed partly due to increased reporting, but also means that the norovirus numbers are exceptionally high this year.

Major infectious diseases – England’s update, week 6

A new week, a new update. TL&DR: Not much change – Covid increases but perhaps not as fast as expected, Scarlet fever is still a problem, but flu and RSV are continuing on a downward trajectory. Norovirus cases are very high again.


Following the recent trend, the Covid numbers are rising in England and in Scotland. The rise in hospitalisation is perhaps not as fast as I expected last week, so hopefully, the wave will be self-limiting.

Looking at the age structure of reported cases, the fastest increase is in 15-19 year-olds and 75-84 and 85+. But all age classes (except 0-14 yo) are increasing.

Deaths typically lag cases by 2-3 weeks, so I expect the increase to translate here soon. I started a new wave for comparison with the previous ones, but this graph will probably require more tuning.

Note that the last 7 data points (red crosses) will likely be revised upwards.


As expected, the flu numbers are decreasing, although – again as expected – the decline is slowing down. The season might still surprise us, but I do not expect a significant outbreak here this year.


Similar to influenza, the RSV season seems to be largely over. The decline is perhaps not as fast as one would wish for, but there is nothing unusual here and we should soon see very small numbers.

Scarlet fever

High levels continue, compared to pre-pandemic years. I expected the numbers to start picking up a bit (following the pre-pandemic trends) but the number of cases seems to be pretty stable. I do not expect them to drop significantly, but neither expect a large increase.


The Norovirus cases are up again. The report says they are “within the overall historical range reported in the decade before the COVID-19 pandemic.” But they are well above the 95% confidence interval based on the last 5 seasons.

The report again puts this increase onto increase reporting for 65+ year-olds. This cannot be simply due to a sudden increase in reporting efficiency but instead due to the spread in the older population, perhaps care homes or hospitals.