Quanta and Qualia: A blog about things and perceptions

Estimating your COVID-19 risk

[Disclaimer: these are my own informal calculations based on my inexpert impression of the science and data, which themselves seem still quite uncertain. They’re meant to encourage you to look at the current data and do your own.

Updates September 14, 2020.

  1. After writing the original post, I found a nice article by Tim Harford that spells out the basic risk estimate calculation.
  2. The figures below have been updated with the most recent ONS and ZOE estimates (as of 14th September). ]

It’s difficult to know what’s worth doing, or not doing, to reduce the risk of covid-19 infection. Here’s a way of cutting through all the uncertainties and getting an estimate of your actual risk levels. I’ll give current figures for England; obviously the method works in any region where reasonable infection rate estimates are available.

First, find the current estimated daily infection rate for covid-19. Early September estimates for England from the ONS are ~3200 per day for people living in households (i.e. not care homes or hospitals). The ZOE Covid-19 Symptom Study give ~4220 per day; this is for the entire UK and appears to be for all settings (including care homes and hospitals). All of the figures come with error bars; for example, the ONS 95% confidence upper bound is ~4600 per day. The figures currently seem to be increasing; the highest current estimate (that I’m aware of) is that they’re doubling every 7-8 days at present; other estimates suggest a lower rate of increase.

Assuming you’re not in a hospital or care home, you might thus conservatively go with twice the latest figures, i.e. roughly 8000 per day, for today. The population of England is 55 million, and the numbers in care homes (<500000) and hospitals (smaller) are small fractions of that. Call the residential population 50 million, rounding down.

So, if you’re a typical resident of England exposed to typical covid-19 risk, your risk of infection is ~ 8000/50000000 = 1/6250 per day. Annualized — if the risk were the same every day for the next year — this gives a risk of about 6% of infection.

Your risk of dying from covid-19 if you’re infected depends on your age, sex, and health. For most people it’s not that high. Estimates of the overall infection fatality rate vary; if we take it to be 0.6% then the average English resident’s annualized risk of death from covid-19 is the product, 6% x 0.6 %, about 1 in 2700. That’s about 1/25 of the overall mortality rate. Your covid-19 death risk might be higher, if you’re old or have existing conditions — but so will your overall death risk, and they roughly scale together. If you die in the next month, unless you’re very atypically at risk, it’s pretty unlikely it’ll be of a covid-19 infection you contracted today.

Of course, the risk won’t be the same every day for the next year. There may be a very serious second wave. The current annualized risk is only a good guide for decisions right now. If you’re behaving like a typical English resident, in a typical environment, it looks as if your risk today is quite low. If you’re following government guidance, keeping social distance, wearing masks if you go shopping or on public transport, and don’t have a job that exposes you to atypical risk, your risk is very likely lower than the figures above. Most non-mandated risk avoidance measures — such as disinfecting groceries or wearing masks outdoors when not socializing and not in crowded areas– will probably not greatly reduce your already low risk. Most experts suggest that the large majority of infections come from indoor exposure to airborne droplets or aerosols. The data aren’t solid, but my sense is it’d be surprising if 10% of infections, even now that we’re aware of the indoor risks and taking countermeasures, come from other sources. So it’d be surprising if disinfecting groceries and non-social outdoor mask-wearing reduced the average risk of infection by more than an annualized 0.6%, or the average annualised risk of death by more than 1 in 25000.

Are these extra countermeasures nonetheless worthwhile today? The emerging consensus on mask-wearing seems to be that it’s more for social good than for individual benefit: my mask protects you much more than me. If a large enough proportion of people wear masks when near others, then, models suggest, the transmission rate can be significantly reduced — plausibly by enough to mitigate a second wave. The inconvenience isn’t that great, and it’s probably a habit we should get used to when we’re anywhere around people. Wear-masks-in-public is an easy rule to communicate, follow and enforce; wear-masks-in-shops-transport-sufficiently-dense-crowds-and-less-than-fleeting-conversations, not so much.

The case is much less clear for grocery disinfection and the like. Rationally, we should put finite prices on our lives and on our time. A quick short-cut (ignoring future discounting and quality of life weighting) is to convert everything into time. A minute a week disinfecting groceries means investing about 1/10000 of the year to avoid at most a 1/25000 risk of death. If your remaining life expectancy is more than 2.5 years, that’s perhaps worth it — but unless you expect to live for 75 more years a more realistic 30 minutes a week perhaps isn’t. (If you’re very young, you might expect to live for 75 more years, but your death risk will be much lower. So you need to be optimistic, not just young, to make it worthwhile on this estimate.) But there are other costs: covid-19 also carries risks of serious and prolonged illness and perhaps lifelong loss of quality or life and lower life expectancy. Maybe the true cost of those risks is several times that of the death risk: my impression is that we just don’t know at present. Still, that might tip the balance towards grocery disinfection. There’s also some social benefit in personal risk minimisation: if you lower your risk of infection, you lower your risk of spreading. On the other hand, assigning 10% of total infections to infected groceries may well be far too high. If grocery disinfection brings you peace of mind, perhaps it’s worth doing, but I’d try not to worry about occasional lapses.

Tl;dr right now, in England (and the rest of the UK), unless you’re especially vulnerable or exposed, or live in a hotspot region where additional lockdown measures are in force, I think your personal covid-19 risk is still low today. Protect others with masks by all means; follow government guidelines, and you’ll be very unlucky to get ill today. But everything depends on the infection rate; I would follow it and reevaluate weekly.

1 Comment

  1. adriankent

    Testing comments

Leave a Reply

Your email address will not be published. Required fields are marked *

© 2024 Quanta&qualia

Theme by Anders NorenUp ↑