The "hospitalised for Covid" part of the sample group is introducing a lot of bias. Fit, young and healthy people are much less likely to end up in hospital. So it makes sense from the first that the group is suffering more problems than a general population control group.
Over a mean follow-up of 140 days, nearly a third of individuals who were discharged from hospital after acute covid-19 were readmitted (14 060 of 47 780) and more than 1 in 10 (5875) died after discharge, with these events occurring at rates four and eight times greater, respectively, than in the matched control group. Rates of respiratory disease (P<0.001), diabetes (P<0.001), and cardiovascular disease (P<0.001) were also significantly raised in patients with covid-19, with 770 (95% confidence interval 758 to 783), 127 (122 to 132), and 126 (121 to 131) diagnoses per 1000 person years, respectively. Rate ratios were greater for individuals aged less than 70 than for those aged 70 or older, and in ethnic minority groups compared with the white population, with the largest differences seen for respiratory disease (10.5 (95% confidence interval 9.7 to 11.4) for age less than 70 years v 4.6 (4.3 to 4.8) for age ≥70, and 11.4 (9.8 to 13.3) for non-white v 5.2 (5.0 to 5.5) for white individuals).
According to the paper, this control group was picked because, essentially, that’s what the authors wanted to choose:
> We selected controls from the general population rather than matching to non-covid hospital admissions to determine the increased risk after hospital admission for covid-19 versus no hospital admission for covid-19 (that is, compared with the expected risk for people with similar personal and clinical characteristics in the general population).
> Individuals were included if they had a hospital episode from 1 January to 31 August 2020 with a primary diagnosis of covid-19
(from the full paper)
People who present once for one thing are probably more likely to present a second time: either because they have a lower threshold for seeking help, they have overall poorer health and so present more often, or because they have a referring primary doctor who is biased towards admitting over treating outpatient.
However, I don't think it can explain the far higher death rate.
Isn't that a clear indication that there's something besides "general trends in old people" going on here?
Looking at the tables, you might get the idea that COVID hospitalization causes a lot of diabetes. The way they arrive at that is to count the diabetes diagnoses made after COVID admission.
However to me it seems far more likely that a diabetic without a diagnosis (which is common) gets hospitalized with COVID, then gets their diabetes diagnosis in the ensuing medical surveillance. Having an undiagnosed liver or heart issue is also common.
(There are other good questions to ask too, of course.)
Why deviate?
They do justify their choice in the actual paper (e.g. see the end of the "Study population" section).
(It's also interesting to compare COVID hospitalizations to other ones, say from a heart attack or allergic reaction, but those are different questions. For example, maybe a COVID hospitalization is less bad than one due to a heart attack, or worse than an allergy. Or maybe any hospital visit is just bad period. But given many people got hospitalized by COVID this year - something that never happened in the past - we need to know what that event cost them, that's the point of this study.)
How do you then control for those with a chronic condition that is undiagnosed? If somebody is hospitalized, they'll perform all sorts of tests on them, that's how many people get their diabetes diagnosis in the first place.
Looking at the charts, you might get the idea that there's a staggering increase in new-onset diabetes following COVID infection, which doesn't seem plausible to me. Yet, right next to that, you see damage in heart, liver and kidneys, and this gets picked up uncritically as an outcome of COVID. The hypothesis that COVID causes organ damage is already well-established, so this just confirms what we expect. What we don't expect, we just ignore, even though it might point to a flaw in our methodology.
But we know that COVID admittances caused a big increase in the total number of people hospitalized - to the point of overwhelming some health care systems. That strongly suggests that underlying conditions would not have sent as many people to the hospital anyhow if COVID had never appeared and infected them.
It is possible you explain the mechanism, though. Perhaps there are lots of people with undiagnosed underlying conditions, and getting a serious case of COVID is enough to worsen those conditions into life-threatening ones. (Perhaps this only hastens the inevitable for some of them.) If this is the mechanism then both you and the article's hypotheses would be correct.
> Rate ratios comparing patients with covid-19 and matched controls were greater in individuals aged less than 70 than those aged 70 or more for all outcomes
> An alternative approach might have involved comparing outcomes after covid-19 and other hospital admissions; such research has recently been conducted with similar data sources to those in our own study (although with a smaller covid-19 cohort), and comparable rates of organ dysfunction were found between patients with covid-19 and patients with pneumonia who were discharged from hospital in 2019.36 We believe that our study design, where comparisons were made with the expected risk in the general population, was more relevant to public health policy, and complementary to the study that used non-covid hospital admissions as the comparison group. Also, the use of non-covid hospital admissions as the comparison group does not allow estimation of excess morbidity because non-covid admission does not necessarily represent an appropriate counterfactual situation to admission to hospital for covid-19, and the size and direction of the inferences will depend on the choice of control admissions.
I wouldn’t call it the most useful one.
If those who were admitted to a hospital with COVID already were less healthy than those who weren’t before they got COVID (and that’s a given. Higher lung capacity can keep you out of the hospital, for example), I don’t think it is a surprise they still aren’t as healthy when they leave the hospital.
I could imagine COVID being causally implicated in respiratory disease or cardiovascular disease, but diabetes? That sounds to me like the control group wasn't representative.
https://www.cdc.gov/coronavirus/2019-ncov/need-extra-precaut...
https://www.thelancet.com/journals/landia/article/PIIS2213-8...
No, because that is in literally the objective: "To quantify rates of organ specific dysfunction in individuals with covid-19 after discharge from hospital compared with a matched control group from the general population."
I wouldn't even rule out that COVID causes diabetes, but at the same time diabetes is commonly underdiagnosed, as is heart or liver disease.
https://www.icnarc.org/Our-Audit/Audits/Cmp/Reports
Page 29 has a chart for spread of age. Most people (88%) live without any assistance in day to day activity.
> 47,780 patients with covid-19 (4,745 admitted to the intensive care unit and 43,035 not requiring admission to the intensive care unit) were included in the analysis