Curious Cook: Covid-19, contact budgets and variolation

In England, chefs diagnosed with Covid-19 showed higher number of deaths (deaths were calculated based on profession). — AFP Relaxnews

I was surprised to encounter a suggestion from an epidemiologist about establishing what she called a contact budget for Covid-19. On the surface, it may seem feasible that one can calculate some sort of “risks” for various activities so that people can monitor their exposure to the coronavirus during some set period; e.g., a week. The basic idea is that if people do not consistently cross some self-defined contact threshold, then one would be safe (or at least safer) from catching the disease.

With this suggestion, there are four metrics to keep in mind when deciding the required threshold. The first is how much risk one can tolerate. So if a household member has some medical condition, then the required risk level should be lower.

The second metric is the level of physical (and moral) comfort one feels about contracting the disease and passing the disease on. The next two metrics are the amount of contact one needs for work and for maintaining mental health.

So dining out may encounter M risk units, going to a pub accrue N risk units, dining at a friend’s house can add O risk units, etc, and the idea is to keep the total of periodic risk units below the decided threshold, otherwise the chances of catching Covid-19 can become unacceptably high. However it should be noted that if people do not care if they catch the coronavirus, and are also happy to pass the disease on, then this is a pointless exercise.

Sadly, this concept of a Covid-19 contact or risk budget is probably a fanciful fallacy, especially as it is so subjective in the first place. The main reason is we currently do not know how many virus particles are needed to cause a Covid-19 infection.

If it is anything like the previous SARS epidemic, then it may be as few as 280 PFUs, where PFUs (or Plaque Forming Units) are viable active SARS-CoV-2 viruses (as opposed to counting all virus particles, including viruses inactivated or incapable of causing infection).

And anyone can come across a few hundred PFUs or more of the virus during any single injudicious event, especially as the main risk factor is actually encountering an infected person. The risks therefore are not necessarily in the activities themselves, although increased participation in social interactions does hike the chances of contact with infected people.

Note that some infected people can disperse millions of PFUs every few minutes into the air around them, so catching enough PFUs for infection would be practically a certainty if one is in close contact with such people for any length of time.

Another risk factor is contact with items and surfaces which have been contaminated by PFUs of the coronavirus, so hygiene (especially hands) would also be a good precaution. Oddly, the suggested “contact budget” did not include this as a risk.

Contact risk

Still, as we are on the subject of contact risk, it is worth looking at the riskiest professions identified by frequency of human contact and exposure to diseases. As expected, topping the list are medical professionals, especially dentists, paramedics, nurses and care workers. Also flight attendants and hairdressers are particularly exposed to other people and diseases.

Death rates from Covid-19 are highest among security personnel, according to a study in the UK. — AFP RelaxnewsDeath rates from Covid-19 are highest among security personnel, according to a study in the UK. — AFP Relaxnews

Other professions with a lot of proximity to people include waiters, cashiers, retail workers, child carers and fast food workers although their disease exposure is lower, possibly due to better barrier protection options.

However, the above data does not fit in so neatly with actual Covid-19 data in the UK. Note that I am using death rates as there is no data about infection rates by occupation.

In data collated between 9 March and 25 May 2020, the most coronavirus-related deaths occurred in men working in security services (e.g., security guards) with 74 deaths per 100,000. After them were male nurses with 50.4 deaths per 100,000 and men working as care workers/home carers with 50.1 deaths per 100,000. Taking healthcare professionals as a whole, males had a death rate of 30.4 deaths per 100,000.

There were other surprising results, especially for men. Taxi drivers and chauffeurs had the second highest rate of deaths at 65.3 per 100,000, followed by chefs at 56.8 deaths per 100,000. Bus and coach drivers recorded 44.2 deaths and retail assistants had 34.2 deaths per 100,000. Interestingly, men in sales and customer service had a rate of 24.7 deaths, lower than administration and secretarial staff at 26.0 deaths or plant and machinery operators at 30.1 deaths per 100,000.

In all cases, women had a significantly lower incidence of fatalities; for example, only 15.3 deaths per 100,000 were recorded for female nurses compared to 50.4 deaths for males.

So for the “contact budget” suggestion to be practicable, it would seem a lot more data would be needed. And one important factor also seems to be gender.

The data also suggests that dining out sensibly is not a hugely significant risk factor unless one is crammed inside a kitchen with a sick chef.

But one should note that the wide disparity of Covid-19 death rates across occupations mentioned above is based on data at a certain stage of the pandemic in the UK. Such limited statistics is probably not robust enough to derive real risk probabilities of mortality rates versus occupations or type of contacts. Also as mentioned earlier, the real risk factor is close contact with one or more infected people and/or contact with fomites (contaminated objects or surfaces).


Hence, although the contact budget is in itself not a bad idea, it is also not terribly feasible in practice and therefore a touch theoretical. This leads to an October 2020 publication in the New England Journal of Medicine (NEJM) which proposed an interesting hypothesis about variolation via the use of face masks. It is based on intriguing observations about the severity of Covid-19 in various situations.

The word variolation comes from the scientific name for smallpox (Variola), and describes the practice used in the early 18th century to protect people against smallpox. Although fraught with danger, it was a simple procedure which involved rubbing powdered smallpox scabs or fluid from pustules into shallow scratches made in the skin.

According to a recent report, medical ill-effects arising from Covid-19 were reduced when people wore face masks. — ReutersAccording to a recent report, medical ill-effects arising from Covid-19 were reduced when people wore face masks. — Reuters

Quite quickly, people treated in this manner would develop symptoms of smallpox but normally with less severity than if the disease was acquired via respiratory transmission from an infected person. Amazingly, within a few weeks most people recovered after variolation and retained a lifelong immunity to the disease. Of course there were still many failures and the practice of variolation stopped once a vaccine was available after 1796.

The suggestion behind the NEJM paper is that severity of Covid-19 symptoms may be related to the amount of active PFUs of the coronavirus acquired. On the surface, it is plausible absorbing over-large doses of Covid-19 PFUs would cause more severe symptoms, just like pouring more petrol over a wood fire would cause larger flames or even an explosion. Whether this is the case with the coronavirus has yet to be confirmed but the paper ‘Facial Masking for Covid-19 – Potential for ‘Variolation’ as We Await a Vaccine’ documented some interesting correlations.

It is important to understand there is no suggestion variolation via wearing face masks can prevent anyone catching Covid-19. Instead the paper reported observations that medical ill-effects of Covid-19 were lessened due to wearing face masks, and this was probably due to smaller doses of PFUs reaching patients.

In the USA, the percentage of asymptomatic people infected with Covid-19 is estimated to be 40%. These 40% of asymptomatic patients do not suffer any significant ailments unlike the other infected 60% who may be severely affected. However, in situations where universal facial masking was practiced, over 80% of the infected patients were asymptomatic, with only 20% suffering medical disorders.

Other data support these rates. On an infected Argentinian cruise ship where passengers and staff were immediately provided with medical face masks, the rate of asymptomatic infection was 81%. In other cruise ship outbreaks, the ratio of asymptomatic patients was only 20%.

In food processing plants in the US, 95% of workers who tested positive for Covid-19 and wore masks were asymptomatic. — FilepicIn food processing plants in the US, 95% of workers who tested positive for Covid-19 and wore masks were asymptomatic. — Filepic

Statistics from US food processing plants also indicate the mandatory use of face masks have had some effect. Of the over 500 workers infected with Covid-19 in these plants since the introduction of face masks, fully 95% of them were asymptomatic. So although face masks do not prevent Covid-19, they might prevent more serious conditions arising from infection.

As such, in the absence of a vaccine at present, this study offers a possible route to mitigate the impact of the pandemic. Presently, the best options for ordinary healthy people to minimise the consequences of Covid-19 may be: (1) maintain a robust immune system (as explained in earlier articles); and, (2) wear a face mask whenever one is around other people. There is a strong chance variolation via face masks can lessen the effects of Covid-19.


After looking at the data, a personal comment is that it is uncertain the strict lockdowns concerning restaurants in parts of the EU and the UK are so helpful. I can agree with locking down pubs, bars and alcoholic gatherings because people simply become less aware of risks after a few drinks.

However, my point is if restaurants are vectors of transmission, then statistics would have shown many more waiting staff affected by the pandemic, especially as they have to constantly move through different clientele every day.

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