For a first post on this blog, I’ll address a question of immediate local concern: How worried should we be about the local and statewide increase in Covid-19 cases that we’ve seen in the last couple of months? Are we heading back to the levels of hospitalizations and deaths that we saw in the spring? A number of local sources have been sounding the alarm in the last few weeks, including here and here. To anticipate the take-home message: No, we are not heading back to springtime levels of hospitalization and death, because what predicts hospitalizations and deaths is not the number of cases, but the number of cases among older people…and we don’t have very many in Massachusetts.
Let’s start by looking at some data. Below are the Massachusetts data from today’s (12/2/2020) New York Times Covid-19 dashboard.
On the upper left, we see the striking increase in the number of documented Covid-19 cases in Massachusetts in the last two months or so. But the two graphs on the right indicate that there has been only a very slight increase in the number of Covid-19 deaths, and only a slightly steeper rise in the number of hospitalizations. While the number of daily cases in recent weeks has actually exceeded the number during the ‘first wave’ in the spring, deaths and hospitalizations are a small fraction of their numbers in the spring.
The dissociation between cases and deaths is even more striking when we zoom in on Hampshire County. The New York Times now presents county-level data; a screenshot from today is below.
In Hampshire County, there has been a ‘wave’ of cases this fall, like at the state level, but simply no detectable increase in deaths. (The Times does not provide county-level hospitalization data.)
What should we make of this dissociation, whereby cases have increased dramatically, but deaths only slightly, or not at all? Two simple positions could be labeled ‘alarmist’ and ‘denialist’. The alarmist position would hold that the deaths are coming; an increase in Covid-related deaths will take a few weeks to appear after an increase in reported cases. But a delay of a few weeks would not be enough to account for this pattern, as cases began their steep rise in October, at both the state and county level. The denialist position would emphasize that we are identifying more cases only because of expanded testing; we are finding asymptomatic and mild cases that will not lead to hospitalization or death, which we would not have identified in the spring, when testing was quite restricted. But this can’t account for all of the increase in cases; the left two panels of the first figure show that while the number of daily tests has gone up by perhaps 50% since early October, the number of cases has gone up by perhaps 300%.
So, neither the alarmist explanation (the deaths are coming, just wait) nor the denialist position (there’s no real increase in cases, only an increase in testing) can fully explain why we have seen such a steep rise in cases, but without much (or any, in Hampshire County) rise in deaths. This blog is called Covid Balance, after all! Fortunately, this puzzle has a clear solution, though it does not seem to have been much discussed in the media.
Below are two sets of graphs from the Massachusetts Department of Public Health. The left panel of the first figure shows the age distribution for cases reported as of May 29. The left panel of the second figure shows the age distribution for cases reported in the midst of the ‘second wave,’ in the two weeks prior to November 27.
We see a rather remarkable shift. In the first wave in the spring, the average age of individuals diagnosed with Covid-19 was 52, while in November, the average age was 39. But much more important is what is going on at the right edge of the age distribution. In the spring, 14.6% (13882/95271) of all individuals who were diagnosed with Covid-19 were over 80 years old, and 23.6% were over 70. In the two-week period in November, only 3.2% (1149/36194) were over 80 years old, and 7.6% were over 70. It is difficult to overstate the importance of this shift: In the Spring, almost 1 out of 7 Covid-19 cases occurred in people over the age of 80, and almost 1 out of 4 over age 70; in November, it was about 1 out of 31 over age 80, and 1 out of 13 over age 70.
Below are two more figures from the Massachusetts DPH, this time showing the age distribution of Covid-19 deaths in the pre-May 29 period, and in the two weeks prior to November 27.
We see that in both time periods, the majority of individuals who died from Covid-19 were in the 80+ age group. In fact, the percentages of deaths that were among individuals in the 80+ group are remarkably similar: 62.6% in the first period, and 62.7% in the second. People over 70 accounted for 84.9% of all deaths in the first period, and 82.6% in the second. The average age of death was 82 in the first period, and 81 in the second.
How can older adults account for a similar majority of deaths in November and in the spring, while accounting for a much smaller proportion of cases in November? The answer is simple. The risk of hospitalization or death from Covid-19 is so much greater for the elderly that most deaths will be among older people even when the vast majority of cases are among younger people. Below is a graphic from the CDC that illustrates just how extreme are the age-related changes in Covid-19 hospitalization and death rates.
The risk of death from Covid-19 is, according to the CDC, 630 times higher in a person over the age of 85 as in a person in the 18-29 year-old comparison group. This difference in mortality risk is so extreme that the number of cases in younger people is almost irrelevant to the overall number of deaths. If you want to know how many deaths from Covid-19 will occur in a particular area, don’t ask how many cases there are; ask how many cases there are among elderly people.
The simple answer to the question of why there has been little increase in deaths and hospitalizations in Massachusetts during this second wave of cases is that this wave is occurring mostly among younger people. It is certainly an interesting question why this is so, and there are probably multiple contributing factors. One factor might be increased awareness of what I’ve highlighted here, i.e., how the severity of Covid-19 depends on age. This has probably made many older people quite cautious, while at the same time it has allowed younger people to take some risks, as they are aware that getting Covid-19 is not likely to result in serious health consequences, for them personally.
The impressive rise in cases, in Massachusetts and in Hampshire County more specifically, does not indicate that we are heading back toward springtime levels of death and hospitalization. To the contrary, with the current age distribution of cases, where only 3.2% of cases are in people over the age of 80, it would be essentially impossible for the state of Massachusetts to return to the levels of hospitalization and death that we saw in the spring. The age distribution could change, of course, but at present we should be reassured that we are keeping the health impact of Covid-19 low, in relative terms, by doing a fairly good job at preventing the most vulnerable among us from getting it.
Adrian Staub (firstname.lastname@example.org).
Constructive comments welcome. Thanks to Rosie Cowell, Carlo Dallapiccola, and Dave Huber for very helpful discussion.