Covid-19 and Obesity

This report shows that in countries where less than half the adult population is classified as overweight [BMI > 25 kg/m2], the likelihood of death from COVID-19 is a small fraction – around one tenth – of the level seen in countries where more than half the population is classified as overweight. Of the 2.5 million COVID-19 deaths reported by the end of February 2021, 2.2 million were in countries where more than half the population is classified as overweight.

from Covid-19 and Obesity: The 2021 Atlas, World Obesity Federation, released March 2021

The report cites several studies, conducted in various countries, demonstrating that overweight and obese people are more susceptible to severe covid infections, including hospitalization, admission to the ICU, and death. These studies find that overweight people are two to three times as vulnerable to severe covid as are non-overweight people. That makes sense: covid severity is associated with a variety of comorbidities, including heart disease, hypertension, and diabetes, that are more prevalent among the overweight and obese. Obesity has also been shown to compromise the immune response to infection.

Extrapolating from these studies, one might expect that a country in which everyone is overweight would experience a covid fatality rate two to three times as high as a country in which no one is overweight. However, the Covid-19 and Obesity report found a tenfold increase in the odds of dying from covid for countries in which “only” half the population is overweight compared to countries with fewer than half of its people being overweight. Almost certainly some other factors in addition to BMI must account for these inter-national comparisons.

Age has been shown to be a huge predictor of covid fatality, more dramatic by far than the reported differences based on overweight. E.g., the CDC reports that Americans aged 50 to 64 are ten times as likely to die of covid as are those aged 30 to 39. Looking at the Covid-19 and Obesity report’s maps, most of the countries with low percentages of overweight people also have populations with a low median age. Per the report:

The figures are affected by the age structure of national populations and a country’s relative wealth and reporting capacity, but our findings appear to be independent of these contributory factors. Furthermore, other studies have found that overweight remains a highly significant predictor of the need for COVID-19 health care after accounting for these other influences.

An “independent and highly significant predictor”: how much of the between-countries differences in covid mortality can be attributed to overweight, independently of age? According to the report, percent overweight accounts for about a third of the variance in covid mortality when looking separately at countries with low, medium, and high percentages of the population above age 65. That’s sort of informative. However, the authors could just as easily have conducted the same analysis for age as they did for overweight, arriving at a statistical estimate of the relative odds of covid death for old versus young countries. I’d expect that difference to be even more stark than the relative odds of high versus low overweight. Still, it seems plausible based on the research that overweight would account for up to a twofold difference in covid mortality rates between countries, independent of age.

Looking at the report’s maps, nearly all of the countries with low overweight percentages are in Asia and Africa, while nearly all of the high-overweight countries are in Europe and America. That’s true for age as well: most of the countries with young populations are in Asia and Africa. What we need is a multivariate model looking at the across-countries correlations among median age, median BMI, covid infections and deaths per 100K population…


Are We Approaching Herd Immunity?

In a Wall Street Journal opinion piece from 17 February, Johns Hopkins surgery professor Marty Makary claims that the US will reach covid-19 herd immunity by the beginning of April — that’s a little more than a month from now.

According to Makary, by the end of March, 250 million vaccines will have  been administered to Americans. That seems wildly optimistic: so far 63 million shots have been administered, and at the current rate that number would double to around 125 million — half of Makary’s projection.

Makary also asserts that a far greater proportion of the population has been infected by the covid virus than is indicated either by case counts or by antibody testing:

“Testing has been capturing only from 10% to 25% of infections, depending on when during the pandemic someone got the virus. Applying a time-weighted case capture average of 1 in 6.5 to the cumulative 28 million confirmed cases would mean about 55% of Americans have natural immunity.”

That assertion is likely based on a CDC report from June which I quoted in a post at the time:

“‘Our best estimate right now is that for every case that’s reported, there actually are 10 other infections,’ CDC Director Dr. Robert Redfield said on a call with reporters Thursday.”

But that was before testing ramped up in the US. From the beginning of the outbreak through June, the US administered around 250K diagnostic tests per day. Since then, the testing rate has averaged around 1.25 million per day — 5 times the earlier testing  rate, capturing a far higher proportion of infections than previously.

Also in June, the CDC reported preliminary findings from several seroprevalence surveys conducted in the US, suggesting a higher rate of infections, and a lower fatality rate, than prior estimates. Makary’s covid fatality rate is derived from this earlier CDC study:

“The Covid-19 infection fatality rate is about 0.23%. These numbers indicate that roughly two-thirds of the U.S. population has had the infection.”

However, in early July, after all the serology data had been analyzed, the CDC revised their estimates, doubling the fatality rate while halving the prevalence rate. By late July the CDC had increased its estimated fatality rate further, to around 0.6 percent, which for 500K deaths to date would put the US population prevalence at around 25 percent. Other epidemiologists converge on or near this updated CDC covid fatality rate estimate for the US; so do I.

Makary observes that antibody tests “almost certainly underestimate natural immunity.” That’s almost certainly an overstatement. An August BMJ editorial speculated that finger-stick antibody tests, which identify IgC antibodies, might undercount IgA antibodies. However, a Mass General study found that covid IgA antibodies spike soon after infection and then decline even more rapidly than IgC antibodies. A relatively straightforward study could evaluate the sensitivity of antibody tests over time: identify a group of people who have tested positive on a covid diagnostic test, then administer antibody serology tests to them over the next several months to determine whether and when their antibodies are no longer detected. A number of US universities regularly test their students and staff for covid infection regardless of symptomatology: it would be easy to sample a subset of these test-positives and follow them over time.

Makary stakes his strongest claim in his very first sentence:

“Amid the dire Covid warnings, one crucial fact has been largely ignored: Cases are down 77% over the past six weeks.”

He’s right: covid infections have dropped precipitously so far this year. Makary insists that the rapid decline “can be explained only by natural immunity.” Granted, the reduced infection rate is puzzling. There’s no evidence to suggest that Americans have suddenly gotten religion about social distancing and masking up and self-quarantining. If, as Makary contends, two-thirds of Americans have already been infected by the virus, the rate of viral contagion would be dropping for lack of new bodies to infect.

However, rates have also been dropping just as rapidly in other countries with lower covid prevalence rates than the US. Cumulatively, about 1,500 per million population have died of covid in the US; using Makary’s 0.23% fatality rate that comes out to 65% of the American population, as he says in the article. In Canada about 570 per million have died of covid. Per Makary that would translate to around 25 percent of the Canadian population having been infected — far from herd immunity — and yet Canada’s infections have been dropping as fast as the US’s. Ireland: 830 deaths per million, 36% infected, infections dropping even faster than in the US and Canada. There are others.

To be sure, some countries in the world are approaching covid herd immunity. Using the more widely accepted 0.6% fatality rate and adjusting downward to accommodate the relatively young median age of the population compared to the US, around 64% of Bolivians have been covid-infected. If Makary’s 0.23% fatality rate were used instead, then around 166 percent of the Bolivian population would have already had covid. Peru, Colombia, Mexico, Ecuador, Panama, Guatemala, South Africa, Iraq — all would have covid prevalences over 100 percent by age-adjusting Makary’s fatality rate and prevalence estimates.

More compelling still are infection rates of subpopulations within the United States. In my most recent post I estimated that 67 percent of Hispanic Americans have already been infected by the covid virus. In calculating this estimate I used published death counts and a 0.65% fatality rate, age-adjusted for the Hispanic American subpopulation. Sixty-seven percent infected: that’s the number that Makary attributes to the US population as a whole, creating herd immunity and triggering the rapid nationwide decrease in case counts. But what if it’s only the Hispanic Americans who have passed the herd immunity threshold: could a rapid decrease in new infections among that subpopulation of Americans account for the overall decrease in America as a whole? Yes. According to my calculations, Hispanics have accounted for 60 percent of all the covid infections in the US. In contrast, I calculate that only 8 percent of white Americans have been infected. Social distancing isn’t just behavioral; it’s also structural, with white Americans buffered from their Hispanic compatriots by fairly rigid racial, cultural, residential, and economic boundaries.

As Derek Thompson observes in a 17 February Atlantic article:

“America’s seroprevalence—that is, the number of people with coronavirus antibodies from a previous infection—is not randomly distributed across the country. Instead, immunity is probably concentrated among people who had little opportunity to avoid the disease, such as homeless people, frontline and essential workers, and people living in crowded multigenerational homes… What I’m describing here is not herd immunity… But it is partial immunity among the very populations that have been most likely to contract the disease, perhaps narrowing the path forward for the original SARS-CoV-2.”

Perhaps similar class hierarchies are at work in Canada and Ireland and other countries with declining infection rates, “narrowing the path forward” through widespread contagion among the most vulnerable sub-herds.