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Herd Immunity or Social Distancing? A Combination, Actually

Herd Immunity or Social Distancing? A Combination, Actually

Sunday, 25 October, 2020 - 05:45

When you listen carefully enough and long enough to a variety of epidemiologists, you can start to see that their pandemic strategies are converging – or at least complementary.

This might seem surprising after the angry flap over the Great Barrington Declaration, which pushed a herd immunity strategy. A group more representative of the mainstream scientific community then presented a scathing response they called the John Snow Memorandum. What if these two approaches have more in common than it might appear?

The essence of the herd immunity strategy is to take advantage of the observation that Covid-19 is a thousand times deadlier in people over 60 than in people under 30. In principle, if you could find a way to protect all those older people, then encouraging younger ones to go on with life could lead to so-called herd immunity — a state in which enough people are immune to hold the disease in check.

Opponents equate herd immunity with running millions of people off a cliff. But there’s a reason it’s caught on with some members of the public. It is, at long last, a strategy. That puts it far ahead of the Trump Administration’s non-strategy of doing almost nothing and telling people the disease will go away.

Harvard epidemiologist Marc Lipsitch, who has signed the Snow petition, said in a press conference this week that the big objection to the herd immunity strategy is that nobody has figured out how to protect the vulnerable. Even now, the virus is again creeping into nursing homes despite best efforts to protect residents. The objection, in other words, is not that thousands or millions would die if the strategy worked, but that we’d see additional deaths if it failed, and that it’s likely to fail.

On the other hand, trying to essentially eradicate the virus — as in New Zealand — is also extremely unlikely to work in a country the size of the US Social distancing and mask-wearing can slow the spread of the disease, but won’t make it go away. And a hodgepodge of state lockdowns, rules and restrictions haven’t offered the public anything like a goal or a timeline. Americans deserve better.

Here’s the thing: the more precautions we take, the lower the level of natural immunity we need to slow the virus.

This is something I learned from interviewing epidemiologists earlier in the pandemic, become herd immunity had become such a point of contention. The percentage of immune people needed to put the disease on the retreat depends on the much-discussed variable R0 - the average number of people each infected person passes the virus to. The higher the R0, the more transmissible the disease, and the higher percentage of the population needs to be immune to slow it down.

If you lower R0 through some degree of social distancing and banning potential super-spreading events, you effectively decrease the fraction of immune people needed to get case numbers to decline. The inverse works too — the more immunity you have, the less suppression you need in order to start watching numbers go down.

Mainstream scientists acknowledge that some degree of herd immunity has already helped. “There’s no doubt if 20, 30, 40% of people are infected, that will slow down transmission … I’m sure there are pockets of immunity that are having an effect,” said Lipsitch. Two prime examples are Florida and Sweden, he says, both of which have skipped prolonged shutdowns and mask mandates and yet both watched earlier peaks decline for a time. “It’s not nonsense to say there’s some herd immunity right now.”

Scientists are still working out how many people need to become infected before herd immunity starts to help contain the virus. The models used early in the pandemic assumed that everyone is equally susceptible to the disease and everyone mixes with an equal number of contacts – a condition under which herd immunity alone wouldn’t cause a decline until 60 or 70% of the population gets infected.

Newer models assume some people are much more prone to get sick than others, and some are much more likely to spread the disease than others. If the most infection-prone are now disproportionately immune, herd immunity kicks in sooner - before even half the population becomes infected.

Lipsitch says we have to consider the sustainability of public health measures to suppress the disease. Otherwise people are being asked to sacrifice jobs and educational opportunities and live more isolated, lonely lives with no discernible goal or end in sight.

The bottom line is that the US needs a strategy rather than an ever-shifting set of rules. And maybe a place to start is acknowledging that social distancing and herd immunity are complements, not opposites.


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