According to this article, the risk of something at least as bad as COVID is 27% over 10 years (which translates to an average annual probability of about 3%). https://www.unmc.edu/healthsecurity/transmission/2023/04/18/disease-forecasters-are-convinced-theres-a-27-chance-of-another-covid-like-pandemic-within-10-years-but-experts-believe-theres-a-silver-bullet/

See also: "Our estimates show that in any given year, there’s a 4.2 percent probability of a respiratory pandemic causing ~10 million deaths". https://www.cgdev.org/blog/how-big-risk-epidemics-really

@Liedholm The WHO has declared 2 pandemics in the past 20 years (the other being the 2009 influenza pandemic, which caused less than a million death), along with a few other PHEIC.

@AliceMcBob Thanks for pointing that out! It is kind of tricky to try to compute the frequency using a BOTEC with so little data though.. For instance, the WHO has declared two pandemics so far, but has existed since 1948. So if we took the whole existence of the WHO as a reference point, the risk would be closer to 1/38 rather than 1/10 for a given year. Anyway, I'm not saying that either of these numbers should be followed blindly!

I feel like I wanna listen to Baba O'Riley now for some reason

@Liedholm yes, good point. I took 2005 as a starting point since that's when the WHO's modern framework started, and before that the terminology was all over the place e.g. HIV/AIDS being referred to as a epidemic/pandemic interchangeably. I couldn't find an official WHO definition of a pandemic, but imo any of 1/76, 2/76, 3/76, 4/76, 5/76, 1/20, 2/20 could be argued to be the base rate depending on your choice of reference class (e.g. does the word pandemic have to be used in an official statement? internal documentation? did COVID change the WHO's internal criteria for using the word "pandemic"? etc)

Looks like there's an official definition of a pandemic as of last year:

1. it must be a Public Health Emergency of International Concern (PHEIC). A PHEIC means an extraordinary event which is determined (i) to constitute a public health risk to other States through the international spread of disease; and (ii) to potentially require a coordinated international response;

2. being of communicable disease nature;

3. having or at risk of having wide demographical;

4. exceeding, or is at high risk of exceeding, the capacity of health systems;

5. causing, or is at high risk of causing, substantial social and/or economic disruption etc.; and

6. requires rapid, equitable and enhanced coordinated international action etc