To figure out what’s going on with Covid-19, a good place to start is in the sewers. Since early in the pandemic, the Massachusetts Water Resources Authority has been testing inflows to its Deer Island Wastewater Treatment Plant in Boston Harbor for the virus. The readings provide a remarkably detailed picture of the course of the pandemic — in Boston and its suburbs, at least.
Epidemiologists still aren’t absolutely certain what these wastewater readings from Boston and elsewhere mean. A bunch of studies have been conducted comparing them with other measures of Covid prevalence, and the overall conclusion seems to be that virus concentrations in sewage do a pretty good job of predicting and reflecting the spread of Covid-19 but that the topic needs more study. I suspect that the Boston-area readings, which are unique in being available in an almost unbroken series back to mid-March 2020, provide a much more reliable measure of actual virus prevalence than confirmed-case numbers based on Covid tests, which were barely available in spring 2020 and are now mostly conducted at home and never reported to authorities.
One thing the sewage readings show is that last winter’s omicron wave was Covid’s peak so far and by far. Another is that the disease was much more prevalent during this winter’s wave, which crested around New Year’s Day, than during the Boston area’s initial encounter with Covid in 2020. Zooming in on April 2020 and December 2022 makes that clearer:
At first glance, this is a little discouraging. After almost three years, in an area where an estimated 95% of the population has received at least one Covid-19 vaccine dose, far more people appear to have become infected with the virus this winter than in the scary days of spring 2020, with the viral signal running from two to nine times stronger in December 2022.
There’s an encouraging side too, though. More than 2,000 Covid deaths were reported in April 2020 in the three counties (Middlesex, Norfolk and Suffolk) that send their sewage to the Deer Island plant; about 200 were reported in December 2022. That’s still a significant number, and with about 93% of Massachusetts Covid deaths in December among those 60 and older, it’s an indication that more needs to be done to protect the elderly and get them life-saving medications. But it’s also an indication that because of vaccinations, previous exposures to Covid-19 and changes in the virus, the disease has become much, much less deadly than when it first arrived.
Here’s the long view, using statewide Covid death estimates by date of death from the Massachusetts Department of Public Health because they’re available on a daily basis and, with Boston-area counties making up almost two-thirds of the state’s population, they should mostly reflect the same virus waves seen in the sewage data.
In April 2020, Covid killed many people in eastern Massachusetts, which was among the US regions hit earliest and hardest by the first wave of the disease. Then the virus receded quickly. Given how much higher the wastewater viral signal climbed in subsequent Covid waves, it seems highly unlikely that this spring 2020 decline was caused by so-called herd immunity, in which so many people have already had the disease that it stops spreading. Instead, it was probably mostly the result of people in Boston and its suburbs hunkering down at home, with warmer weather helping out some, too. The “crush the curve” strategy of the early days of the pandemic actually did crush the curve, although it didn’t eradicate the virus.
Since the arrival of the omicron variant in late autumn 2021, the Boston-area viral signal has mostly stayed well above its April 2020 peak, which can probably be attributed to the more infectious and immunity-evasive nature of omicron and its descendants and the fact that people aren’t hunkered down at home anymore. But, again, the disease has become much less deadly. Hunkering down at home is much harder to justify now than it was in April 2020.
Another way of putting it is that the huge difference between the initial reaction to the virus and the current stance seems more or less justified by changes in its virulence. For an immunologically naive population Covid-19 was (and probably still is) a dangerous disease, much deadlier than seasonal flu and other common respiratory ailments. Now that almost everyone has been exposed to it through vaccines, infections or both, it’s much less dangerous — although so infectious that it’s harder to avoid, too, which is problematic for those still at high risk.
None of this is exactly news, of course. But it is somehow reassuring to see it backed up by what has been turning up in Boston’s sewers.