Has COVID led to an increase in all-cause mortality? A look at US data from 2015 to 2020

Has COVID led to an increase in all-cause mortality? The figure below shows mortality data in the US for the 2015-2020 period. At the top chart are the absolute numbers of deaths per 1000 people. At the bottom are the annual change percentages, how much the absolute numbers have been changing from the previous years.

As you can see at the top chart the absolute numbers of deaths per 1000 people have been going up steadily, since 2015, at a rate of around 10 percent per year. This is due primarily to population ageing, which has been increasing in a very similar fashion. Since life expectancy has been generally stable in the US for the 2015-2020 period (), an increase in the number of deaths is to be expected due to population ageing.

What I mean by population ageing is an increase in the average age of the population due to an increase in the proportion of older individuals (e.g., aged 65 or more) in the population. In any population where there are no immortals, this population ageing phenomenon is normally expected to cause a higher number of deaths per 1000 people.

Now look at the bottom chart in the figure. It shows no increase in the rate of change from 2019 to 2020. This is not what you would expect if COVID had led to an increase in all-cause mortality in 2020. In fact, based on media reports, one would expect to see a visible spike in the rate of change for 2020. If these numbers are correct, we have to conclude that COVID has NOT led to an increase in all-cause mortality.

Do COVID cases spike in cold weather? A look at effective reproduction rates from October to December 2020

Do COVID cases spike in cold weather? In a previous post () I argued that COVID cases may in fact go down in cold weather, due to compensatory adaptation (). The figure below shows the effective COVID reproduction numbers () in various states in the USA from the middle of October to the middle of December 2020. As you can see, as the weather patterns have become colder from October to December, COVID transmission rates have been generally improving.

While the risk of COVID transmission may go up with cold weather, which tends to lead to an increase in indoor activities and potentially higher transmission, people react in a compensatory way. This feedback loop may lead to results that are unexpected and surprising, as we can see here.

Do COVID cases spike in cold weather?

Do COVID cases spike in cold weather? To answer this question, it may be instructive to look at COVID figures for Brazil and the US, because these two countries have had similar responses to the pandemic (); and have opposite weather patterns – when it is hot in the US, it is cold in Brazil, and vice-versa. This applies particularly to the most populous areas of the two countries.

At the time of this writing, we had the following approximate numbers for Brazil and the US. Brazil – population: 209.5 million, COVID cases: 4.12 million, and COVID deaths: 126 thousand. US – population: 328.2 million, COVID cases: 6.26 million, and COVID deaths: 188 thousand. The two graphs below show the two following ratios: cases-to-population, and deaths-to-population.

In the last several months since the pandemic hit both countries, it has been generally cold in Brazil and hot in the US. Given this, these ratios are too close to support the assumption that COVID cases spike in cold weather. So, a reasonable answer to the question posed in the title of this post is: probably not.

This brings to mind another question: would indoor activities, such as restaurant dining and movie theater attendance, lead to spikes in COVID cases?

Well, the idea was that cold weather would lead to more indoor activities ...

While risk of infection may go up with cold weather and indoor activities, people react in a compensatory way. This feedback loop may lead to unexpected results.

Herd immunity

The figure below is adapted from an article published in 2011 by Fine and colleagues (). The article discusses the concept of “herd immunity”: individuals with immunity against a disease act as a “shield” for the community, slowing or stopping the spread of the disease.

At the top of the figure, the number of infected individuals grows exponentially, until a certain number of individuals with immunity is achieved. At the bottom of the figure, those with immunity “absorb and kill” the infectious agent, without passing it forward – significantly limiting the progression of the disease.

This illustrates the likely impact of vaccination in cases where immunity being acquired through full infection is problematic, such as with COVID-19. In these cases, vaccination would slow or stop the spread of the disease, even if only a proportion of the community is vaccinated.

That is, until the infectious agent mutates!

China’s relaxing of COVID-19 social distancing policy after containment appears to have worked

The graphs below summarize key results from a study published in early 2020 by Ainslie and colleagues (). Dr. Ainslie is in the Faculty of Medicine, School of Public Health, Imperial College London. The study looked at within-city movement, as a proxy for economic activity, and how that movement has influenced the numbers of new cases of COVID-19 in various areas, after initial containment.

As you can see, after initial containment is achieved, within-city movement (measured through a “Movement Index”) seems to be uncorrelated with new COVID-19 cases; or somewhat negatively correlated, as the authors note.

This surprising and counterintuitive outcome may be due to people becoming much more cautious about social interactions.