Immune, But For How Long?

I believe we can now be confident that immunity from COVID-19 lasts for at least 6 months, whether an infection becomes symptomatic or not.

During much of the pandemic, all kinds of doomsayers and worry-warts have cried about COVID immunity disappearing. Here’s a paper that shows recovered patients that never developed symptoms were far more likely to lose their antibodies within 3 months than patients that got sick. With asymptomatic infections currently estimated at 40% of all infections, that could be a real concern.

We’ve also now seen several documented cases of legitimate reinfections:

Recently, though, a team of Chinese scientists published a paper that studied symptomatic COVID patients, showing that antibodies were still detectable 6 months after infection. But that still didn’t answer the question of lasting immunity in asymptomatic patients.

I believe we can safely say immunity will last for 6 months or longer in almost all people who are infected by SARS-CoV-2. Here’s why.

Back in April/May, I had a series of python scripts I ran daily which generated charts from curated COVID data in the U.S. One major phenomenon I noticed was that the trends in deaths/cases in New York were diverging greatly with the rest of the nation. New York deaths steadily dropped, while deaths in the rest of the nation continued to increase for quite a while before they finally peaked.

Given everything we knew at the time, I found this surprising at first. How was it that New York was able to get control of this, given all their inherent disadvantages, while the virus continued to spread around the rest of the country? Were people in NYC social distancing more? Was Andrew Cuomo some kind of hero?

No. Andrew Cuomo is neither a hero nor a competent governor. My hypothesis at the time, which I now believe has shown to be true, was that NYC had reached a level of public immunity necessary to keep the Rt of the virus below 1.0 (update: some more evidence on this). This is not to say they actually reached true herd immunity (which is what I originally thought before any of the seroprevalence studies were published). If NYC were to go back to completely normal, they would almost certainly see a surge in cases. But with some levels of social distancing, they have enough immunity to keep cases from surging.

Here’s a chart of NYC cases over time, smoothed by applying a 7-day moving average:

Now, some people will continue to argue that the real reason cases were brought down and remain low is that NYC is still locked down and citizens are still exercising extreme social distancing. What does the mobility data for NYC, provided by Apple, say about that?

Note that mobility in NYC hit bottom about a month before cases peaked. After the peak, as cases continued to decline, mobility continued to increase. (A couple of caveats: it would be better if the mobility were measured as year-over-year, instead of indexed to Jan 1. Also, the cases shown are impacted by testing availability).

What’s especially important is that even 6 full months after the mobility trough, we haven’t seen any real surge in COVID cases in NYC. If a significant percentage of people infected with COVID were to lose their immunity after 6 months or less, symptomatic or not, we’d almost certainly see some surge in NYC by now.

Given that the number of genuine reinfections worldwide has been limited so far, I’m now guessing that immunity will last a year or more. Of course, that’s less certain at this time.

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