Pandemic probability
2021-Jan-05How probable is a new pandemic? I try out some armchair epidemeology.
I estimate there is 8% probability of the WHO declaring a new pandemic in any given year. I estimate it's 7% in 2021.
Historically, there is an 8% chance of the WHO declaring a new pandemic.
An on-going pandemic does not stop a new one starting, but COVID-19 controls may help stop an outbreak, like the it has made this year's flu activity much lower.
There are reasons to be pessimistic: more intense farming, anti-vax sentiment, a lot more travel, antibiotic resistance, and climate change. On the other hand, there are good reasons to be optimistic: better monitoring, better sanitation, existing vaccines for cholera and yersinia pestis, and better vaccine technology,
Base rate
I found 17 pandemics in the last 220 years, which puts the annual base rate at 7.7%, with a 95% CI (Binomial distribution) at 4.6% to 12.1%.
| Year of emergence | Name |
|---|---|
| 1817 | First cholera pandemic |
| 1826 | Second cholera pandemic |
| 1846 | Third cholera pandemic |
| 1855 | Third plague |
| 1863 | Fourth cholera pandemic |
| 1881 | Fifth cholera pandemic |
| 1889 | 1889 - 1890 flu pandemic |
| 1899 | Sixth cholera pandemic |
| 1915 | 1915 encephalitis lethargica pandemic |
| 1918 | 1918 influenza pandemic |
| 1929 | 1929–1930 psittacosis pandemic |
| 1957 | 1957–1958 influenza pandemic |
| 1961 | Seventh cholera pandemic |
| 1968 | 1968 flu pandemic |
| 1981 | HIV/AIDS pandemic |
| 2009 | Swine flu pandemic |
| 2020 | COVID-19 pandemic |
This does not appear to be changing much. In the last 110 years we've had 9 pandemics (~7.5%), and only 3 in the last 50 years (~6%).
At the time of writing, there are 3 on-going pandemics:
- The seventh cholera pandemic (known as El Tor),
- The HIV/AIDS pandemic, and,
- The 2020 COVID-19 pandemic.
Reasons to be optimistic
We have vaccines for cholera and bubonic plague. We can inoculate people in an affected region if there is an outbreak before it becomes a pandemic. Clearly, we're not great at this as we have an on-going cholera pandemic.
WHO has monitoring to detect outbreaks quickly. This is how we knew about COVID-19 quickly. Healthcare workers can react quickly to contain an outbreak. This worked well for zika and ebola.
As COVID-19 has shown, we have incredible vaccine technology. We can create a vaccine new pathogen in months. It then takes a few more months to go through the testing and approval process, totalling 9 months to one and a half years.
We should also see fewer pandemics as sanitation improves. Clean water means fewer opportunities for cholera outbreaks and better vermin control means fewer plague outbreaks. This not absolute or total -- there are places with poor sanitation -- but things are improving.
In 2021 we have one extra reason to be optimistic: COVID-19 restrictions. Less long distance travel, less person-to-person contact, more mask wearing, more hand-washing. These can all help contain a new outbreak and stop it becoming a pandemic. For example, the flu has killed very few people this year.
Reasons to be pessimistic
It took less than 3 months for COVID-19 to make it to 6 of the 7 continents (I think Antarctica is an outlier). Global travel exacerbates the risk of a pandemic because the usual definition includes different community transmission in "regions." Without travel, you can't easily get a disease from one region to another. COVID-19 hasn't stopped travel, but it has reduced it.
The anti-vax movement and general vaccine hesitancy means one of our best tools to control outbreaks are less effective.
We may be creating more opportunities for novel diseases as we intrude further into animal habitats.
Some bacteria, such as antibiotic resistant staphylococcus aureus, are resistant to antibiotics. If a more transmissible bacteria, such as yersinia pestis, developed antibiotic resistance we could see a new pandemic.
Finally, climate change has two effects: Firstly, it causes the mass movement of people and often confines them in unsanitary conditions. Secondly, it allows vectors such as the anopheles mosquito to move into regions it was previously not common. That gives it a new, likely more susceptible, population to attack.
Overall estimate
The base rate is between 5% and 12%, with 8% the most likely base rate.
I found 4 factors that may make a pandemic more likely and 4 that may make it less likely, so I'll call this a tie.
I expect the probability in 2021 is lower as the COVID-19 restrictions can control new outbreaks. I think the 2021 probability is 7%. I think the "general" probability when COVID-19 restrictions end is 8%.