Continuum, binary, or continuum? Yes.
Understanding risk outcomes is essential to understanding decision making.
An acquaintance recently asked me: why all the contrary studies about masks and infections?
His question was in response to a recent article I had shared looking at relative effectiveness about masking and respiratory virus transmission. The upshot was that almost any kind of mask, including cloth masks, reduces viral transmission of Covid-19. A previous, widely reported study from the Cochrane Collaborative had suggested that there was no convincing studies (as of the end of 2022) that wearing masks were of any benefit to prevent acquisition of Covid-19 or influenza.
The answer to the question of why the contrary studies is that while they were all about masks and respiratory viruses, they were different study methods looking at different outcomes. Some outcomes were binary—did or didn’t an infection occur. Some outcomes were on a continuum—how much risk of infection occurring or how sick did someone get after wearing a mask.
When it comes to infectious diseases, it is important to understand what outcome it is that one wants to predict or affect.
My mantra is: risk is a continuum (from 0% to 100%), but infection status is a binary. One either has an infection or one does not. But wait—there is another continuum! The effect of the infection may also be on a continuum, from no symptoms (subclinical) to mild, moderate, severe or fatal.
Those subtleties can be lost in shouting matches, memes and internet echo chambers. There are those who may feel that any infection is a failure of prevention. Others may feel that they can tolerate an infection, but don’t want a bad outcome (hello, vaccines!). Still others may feel that if risk of infection or bad outcome isn’t reduced to zero, then why bother doing anything at all. Those are all different outcomes that may be binary (did or didn’t get infection) or a continuum—risk of getting infection and, if infected, what the outcome is of the infection.
Some studies look at risk of transmission of particles, some studies look at whether an infection occurred (or was acquired) or not—the binary outcome. Some studies look at whether the ultimate outcome occurred—death. When the Covid-19 vaccines were introduced, the primary outcome measured was death, the end of the continuum of infection.
I have had patients tell me, “I don’t want any risk of infection.” My answer is usually, “Did you drive over to see me today? Was it worth the risk of some drunken individual hitting you on the highway?” (This retort worked better in the days before telemedicine.) My point being that we all tolerate some risks to get what we want. We know that the risk of driving somewhere without getting into a collision is a continuum, but there are things we do to mitigate that risk of a collision—don’t drive in bad weather, don’t speed, drive “defensively”. Even doing all those thing, there is still a risk of collision. Seat belts and airbags don’t change the binary outcome of whether a collision occurred. Rather, they reduce the worst outcome on the continuum that occurs after a collision.
There are multiple ways to reduce risk of respiratory disease transmission, some are things that we can do, and some depend on the actions of others. They all have effects. The outcome of those effects can also be on a continuum from negligible to death.
What to do about masks now? If your goal is to prevent your own acquisition of a respiratory infection, then wearing a N95 mask in indoor crowded areas is your best bet. A KN95 or surgical mask will likely reduce your risk of acquisition over no mask at all. How much risk reduction you get will depend on what mask you wear and how long you wear it. Additionally, it may also depend on how much virus is around you. Wearing a mask outdoors, or in a very well ventilated space, adds little benefit. Those are the inputs on the risk continuum.
If your goal is to prevent others from catching a respiratory virus from you, then wearing almost any mask has benefit. Some employers may recommend, or even require, this in work settings where employees crowd together. Some viruses are transmittable for a day or more even before symptoms start, so this can be especially important during times of high transmission in the community.
Vaccines can reduce the risk of an adverse event happening if infection occurred, and may also prevent the risk of transmission to others. Reducing the risk of transmission is especially important in settings where people may lack the ability to have their own robust immune response, such as nursing homes.
If it feels like infectious disease is a casino, well, it is. And it can be worth it to tip the odds in your favor.
Excellent explanation of different outcomes. It’s so helpful when reading about so many covid issues, but also the many medical and scientific articles I read about myeloma. Understanding the the whole outcome is important!