The Who, What, Where, When, (S)why(ne Flu):
Epidemiology is the 'upon' 'people' 'study' (from Greek)- an aspect of public health that brings us all the way from genome sequences of viruses found in wastewater, all the way up to 'how much money should we spend on a vaccine for this disease?'.
Public health essentially concerns itself with a population- it may encounter clinical signs or individual experience, but unlike other biological fields, the 'public' takes a general study, not to predict the lived experience of a select individual, but rather the patterns, risk factors, predictors, interventions, rates, statistics and more that govern our societies, countries, world and beyond.
Public Health encompasses epidemiology as both a noun (a collection of knowledge about disease, both infectious and non communicable, and how risk factors or interventions can prevent or increase rates of ill heath outcomes) and a verb (a toolset to deal with the questions we ask and the answers to face about populational health)
The basic premise is:
- Assess the health of a population
- Instigate or observe an intervention or trend affecting the health
- Evaluate the accidental or intended intervention
- Try to improve population's health
Sounds easy(!)
Epidemiology comes in around the 'assess' part most of all, with surveillance and methods of identification. Epidemiology grapples with the varied, intangible and dynamic aspects of what a disease or disorder is, the aspects and influences of health, and the interdependence of animals, plants, humans, the environment and more.
The technical definition is:
- the branch of medicine which deals with the incidence, distribution, and possible control of diseases and determinants relating to health.
Or what the WHO states:
Epidemiology is the study of how often diseases occur in different groups of people and why. Epidemiological information is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom disease has already developed.
But as we dive into exploring epidemiology together- let us instead use a simplified but all-encompassing definition I prefer:
Epidemiology is asking the 5 W'sof disease.
WHO
WHAT
WHERE
WHEN
WHY
Who is affected? What are the symptoms/disease? Where is it spreading/affecting? When is the time period of ill health? and Why? (what have the individuals affected done that individuals not affected haven't and vice versa).
We determine cause, risk, origin, prognosis, spread and more by asking these questions, to volunteers, patients, their families, governments, international agencies, healthcare workers, policy makers, businesses, the general public and more.
As an example, take the recent surges of SWINE FLU
H1N1 flu is a type of Influenza A virus and it's been infective in humans since 2009, however has also combined with pig, bird and human viral strains to create hybrids.
Let's however don our 'epidemiologist' (in training) hats.
The WHO confirmed on 2nd September 2023 a laboratory confirmation of a human infection of H1N1 Swine Flu in Netherland, in an adult with no medical conditions known to them and no exposure to animals (usually only farmers and animal workers ever saw cases).
How did this happen, what may happen next, and how do we safeguard the health of the public without raising alarm, fear or unneeded disruption?
An outbreak of H1N1 isn't rare or incredibly unknown, but this case was alarming in the demographic risk of exposure to infected animals through direct jobs being missing. How was this individual infected, where and when, what infected him, who else may be and why him?
Some extra context:
This case was the first recorded case of swine flu in the Netherlands in 2023, and one single infection is unlikely to cause an outbreak but the CDC (who responded along with WHO and the local authority) rated the risk from H1N1 in humans at 'potential pandemic' to 'some extent' on the Influenza Risk Assessment, and H1s have the current highest pandemic potential compared to H2 or H5 viruses, and the lack of monitoring for pigs, along with the cases being passed often as typical flu, means some cases may be unreported resulting in more than just one case.
The case was picked up by regular surveillance (notifiable disease reporting and disease mapping for the win!!) but the individual reported no direct contact with pigs (unlike a previous outbreak a few years ago among pig farmers).
As a budding epidemiologist, you decide to take some more data. You ask for contacts, places visited, foods eaten. Your first concern is both what caused this infection but also how many others may be infected. You reach out to anyone in contact with the patient and ask about symptoms, as well as increasing the routine surveillance in the area for H1N1 strains for patients with flu symptoms.
H1N1 Swine Flu appears to cap at 10 day incubation periods, so you follow all 5 named contacts your patient reported, and none showed any issues for the quarantine period. It is unlikely that your patient transmitted it to another human in this case, and your routine swabs showed no other diagnosed tests.
So you have a bit of a relief in that it's not already silently spreading, but you can't prevent it happening again unless you know how it happened. The patient is getting treatment and is isolated, but had on 20 August 2023 (around 2 weeks before you could mobilise, and thus a 10 day incubation meant exposure was around mid August) the patient developed severe fatigue, seizures and acute respiratory distress. You repeat a throat and nose swab which tests positive still for the Influenza A H1N1 strain but you send it for advanced diagnostics at the Dutch National Influenza Centre and find it is negative for A(H1N1)pdm09 which is a new seasonal influenza strain that had human-human transmission due to being a quad-virus with 2 swine, one human and one avian origin viral ancestor. The issue of H1N1 negativepdm09 is still dire but much less transmissible or dangerous than if your patient had that strain. In addition, all other available viral strains from other families were negative, this seemed to be a single virus infection rather than hybrid or joint strains.
However, after 3 days, you manage to get genome sequencing of the exact virus and find a receptor sensitive to 2 available neuroaminidase inhibitors. Well, perhaps the clinicians would know the sensitive drugs or the exact treatment, thus as an epidemiologist you are not involved directly in choosing this treatment, but you would have been the one to send the swabs to variant and genome analysis, as the cost and training needed is at the hands of the public health responce team. So by working together, you may be no closer yet to the cause, but the patient starts improving and recovers by mid September, and there don't appear to be community or close-contact cases. So now you can focus fully on the origin.
Let's reassess:
WHO- an adult patient with no known medical issues or exposure to pigs- you managed to assist in getting better treatment and to isolate contacts, BUT the who should play into 'how can a single virus cause such illness in an otherwise healthy non at-risk patient' and 'how would they get exposed'
WHAT- the receptor sequence may have helped with treatment but you don't have enough mapping to track where these mutations came from to get a chain of infection, you do at least know it's not a more deadly quad-viral version, or a new mutation, but rather for some reason a single viral H1N1 strain infected your patient
WHERE- Netherlands, with no obvious community flu symptoms reported, no specific diseases, no lab test positives, no known risk factors, and no contact to pigs
WHEN- it's been it's now been about 2 weeks since the 2nd of September notification of with a lab test positive (which was reported to WHO and CDS through the Europe Responce and Emergancy Centre within hours) and symptoms began 20 August indicating exposure between 9th to 19th August 2023.
WHY- more evidence needed. What exposure or activity or risk did this person have that the contacts or community didn't to acquire the infection, and also was there anything that increased the potency to cause such severe illness?
You continue research, and there are more cases worldwide, some are amongst animals (pigs, birds), as expected, but some in humans with direct contact (farmers, vets and some healthcare workers) but human to human transmission is still unlikely and no community outbreak has occured elsewhere or a match to the genome you sent to the international joint database which would at least tell you more about the spread.
And yet I have sad but common news for you. As of 5th January 2025 (over a year and a half on), we still have no indication of the cause or the chain of infection that lead to our Netherland Sept 2023 patient. Epidemiologists may still keep an eye on it, but in a world of constant infections and disease, you are not able to find all the answers or continue asking questions. The case is paused, the ill health was brief and isolated, and all in all, despite great unknowns, it is a public health success. If you hadn't helped with finding specifics, you may have missed silent cases, or not helped maintain better adapted treatment, or ensured we have at least a new data point for our international monitoring. We may never know the source of infection, or the exact sequence may resurge there or anywhere else, and then we will point our questions there once more, but for now, there are just too many fires to fight to find the answers to our strange adventure.
However, I leave with one thought. More outbreaks, of many diseases, and even of swine flu in many countries continue. But a few months ago, November 2023, close to home in the United Kingdom, the WHO reported another influenza A infection, of H1N2 strain- from a routine sample analysis of a short ~2 week flu-like infection. The patient was fine but also had no direct exposure to pigs, and yet the variant belonged to one detected in many UK pigs. But this was the first recorded case of H1N2 in humans. Contact tracing and routine swabs continue, but the case seemed (milder but) nearly identical to the demographics of our Netherlands case, with no risk factors that we know, no idea how it started or how it just fizzled out, or what it means for our future.
This particular outbreak variant is so common in many UK pigs therefore is still actively monitored by the WHO emergencies disease outbreak team. The subtypes H1N1. H1N2 and now AH3N2 have become common as subtypes from combinations of swine flu with other accumulated mutations. Since 2018, the UK (and many other countries) have had these seemingly random single human cases (with no risk or contact) pop up and die down, but as some variants become more transmissible, more lethal or more common, the need for surveillance, routine testing, contact tracing, community education and more are needed to ensure the health of our future is as safe as we can make it, from not just this disease, but all disease and ill health that may befall our world.
Disclaimer: I am learning biology, but I have no qualification in it beyond begginer exams, no accreditation of knowledge and will barely scratch the surface. I hold ambition to explore, a curiosity to admit mistakes and be told my misunderstandings, but I also claim no gravity to the blog posts on it beyond light entertainment of 'the basics' and my own discovery. Please comment or get in touch with advice, (polite) critiques and facts, but also do your own research if you attribute anything I write to base your own opinions on. All my knowledge (to the best of my growing but limited ability) is from 'reputable' beginner sources such as CDC, WHO, NHS, UKHSA for policy, statistics or facts- but these sources themselves are limited, imperfect and far beyond my scope to evaluate. All credit goes to my inspirations and learning materials, and all blame or mistakes go to me.
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