Has anywhere in the world reached herd immunity? Possibly, but at terrible cost

Has anywhere in the world reached herd immunity? Some communities may have, but at terrible cost.

Today I’m looking the level of population (or herd) immunity around the world. Early on in the pandemic, there was a talking point, which is creeping back occasionally,  that we should just stand back, and let everyone get infected, to avoid the economic consequences of non pharmaceutical interventions like closing restaurants, and big gatherings. This article describes it well:

Here are the basics. When a large proportion of a community becomes immune to a disease, its spread from person to person becomes substantially reduced. As a result, the whole community becomes protected, not just those who are immune. This phenomenon is often called ‘herd immunity’ but I prefer ‘population immunity’ as a less loaded term.

First how much spread is needed?

The theory reasonably straightforward. Once a significant part of a population is immune to a disease, it is difficult for it to spread, as an infectious person has no-one to spread it to. And what is the proportion? Well that depends on how infectious it is. There are a few different estimates for Covid19. But let’s say the R0 is 3 – ie without any interventions, an infected person will infect 3 others. In that case, a standard epidemiological formula (1-1/R0) estimates the population immunity required to stop spread of gives you a required population immunity of 67%. That formula, though, has big assumptions in it, most notably that every person in the population is equally likely to spread the disease.

If there are superspreaders, it is possible that the threshold reduces. This article sets out how that works:

There are situations when herd immunity might be achieved before the population immunity reaches p_I = 1 − 1/R₀. For example, if some individuals are more likely to get infected and to transmit because they have more contacts, these super-spreaders will likely get infected first. As a result, the population of susceptible individuals gets rapidly depleted of these super-spreaders and the pace of transmission slows down. However, it remains difficult to quantify the impact of this phenomenon in the context of COVID-19. For R₀ = 3, Britton et al.² showed that, if we account for age-specific contact patterns (for example, individuals aged >80 years have substantially less contacts than those aged 20–40 years), the herd immunity threshold drops from 66.7% to 62.5%. If we further assume that the number of contacts varies substantially between individuals within the same age group, herd immunity could be achieved with only 50% population immunity. However, in this scenario, the departure from the formula p_I = 1 − 1/R₀ is only expected if it is always the same set of individuals that are potential super-spreaders.

And this article looks at some modellers who have taken this further and suggest that population immunity could occur with infection rates as low as 20%. This depends on a lot of untested assumptions But the big assumption is that the superspreaders are always the same people – it is something about who they are, rather than their behaviour, which makes them superspreaders.

another group, led by the mathematician Gabriela Gomes of the University of Strathclyde in Britain, accounted for variations within a society in its model and found that Belgium, England, Portugal and Spain have herd immunity thresholds in the range of 10 to 20 percent. “At least in countries we applied it to, we could never get any signal that herd immunity thresholds are higher,” Dr. Gomes said. “I think it’s good to have this horizon that it may be just a few more months of pandemic.”

Other experts urged caution, saying these models are flawed, as all models are, and that they oversimplify conditions on the ground.

But now, as we have second waves of the virus around the world, all those hopeful models are starting to look less likely, as this summary of the arguments explains. If a population infection of 20% was enough to achieve herd immunity, then there shouldn’t be a second wave in Madrid (first wave around 15% infection), in London, in Paris and Marseille, or most recently, Sweden.

The places where I’ve seen the highest levels of infection documented are Mumbai, in India, and Manaus, in Brazil.

In Mumbai, a study showed:

As many as 4,234 individuals were surveyed at random in slums, of which 56.5% were identified with Covid-19 antibodies. In non-slum areas, antibodies were prevalent in only 419 (15.5%) of 2,702 people examined.

In Manaus, in the Brazilian amazon:

In June, one month following the epidemic peak, 44% of the population was seropositive for SARS-CoV-2, equating to a cumulative incidence of 52%, after correcting for the false-negative rate of the antibody test. The seroprevalence fell in July and August due to antibody waning. After correcting for this, we estimate a final epidemic size of 66%. Although non-pharmaceutical interventions, plus a change in population behavior, may have helped to limit SARS-CoV-2 transmission in Manaus, the unusually high infection rate suggests that herd immunity played a significant role in determining the size of the epidemic.

One more group where herd immunity may have been achieved is New York’s Hasidic Jewish community

More than 3,500 people responded quickly to a subsequent survey asking respondents for antibody test results, leading the doctors to estimate that a majority of community members could have antibodies. They estimated that slightly more than 70% of the community’s adults between the ages of 25 and 65 had “been sick with COVID-like symptoms.” Among adults over age 65, they estimated, 55% had been sick.

and that came at a high cost.

That was the beginning. Over the coming weeks, the communities were ravaged by the disease. Death notices were posted hourly on Orthodox news sites. Burial societies worked continuously and ran out of the shrouds required to lay the dead to rest. Funeral homes recruited people with SUVs to transport bodies they lacked the capacity to handle. They had prepared 700 bodies for burial during a period in which they would normally have prepared 100.

In the UK, by contrast, despite the very heavy death toll, the highest local prevalence the authorities have found is around 17%, in London

After making adjustments for the accuracy of the assay and the age and gender distribution of the population, the overall adjusted prevalence in London increased from 1.5% in week 13 to 12.3% in weeks 15 to 16 and 17.5% in week 18.

In the US a study from Pew took a different approach, it asked people whether they had tested positive, or whether they were “pretty sure” they had had Covid19. The results are shown in the diagram here, but in total, 3% of adults surveyed had tested positive and another 11% were “pretty sure” they had had Covid19. So the US as a whole is a long way from herd immunity, even if all of the “pretty sure” people did have Covid19.

And in Sweden, which is most famous for the herd immunity strategy, a study in Stockholm measured past infections. Overall in Stockholm, the past infections were nowhere near population immunity levels.

The prevalence of COVID-19 antibodies on June 17–18, 2020 was investigated in two residential areas of Stockholm, Sweden. Among the residents in Norra Djurgårdsstaden, a newly built upper- and middle-class area of Stockholm, 4.1% of study participants had SARS-CoV-2-specific antibodies, while in Tensta, a highly segregated low-income area, 30% of the participants tested antibody positive.

While it seems likely that there are probably small to medium sized communities in other parts of the world (like the New York Hasidic community) where Covid19 has swept through to the level that more than half of the population has been infected, the continued spread of positively diagnosed cases around the world suggests reaching a point where spread would naturally stop without continued social distancing would take multiple times again as much suffering and death as has happened so far.

Links

At some point, I’ve been intending to have a look at the gendered based outcomes from Covid19, both the disease, the healthcare workers and the flow on economic impacts. But as a taster, here is a link to an article describing the economic situation in Australia  and why it matters to analyse outcomes with a gender lens (as I noted in my review of the fantastic book Invisible Women).

At the Senate Select Committee on COVID-19 on Tuesday, the Treasury confirmed that this downturn is disproportionately negatively impacting women and younger people, who are over-represented amongst those who have lost their jobs. Much of that, the Treasury confirmed, is due to the structures of the industries most affected (arts, recreation, retail, accommodation), areas where employment is dominated by women and younger people.

Life Glimpses

Since March, I’ve been learning french in a group class once a week. The very first class was in person, since then it has been via zoom (and surprisingly effective). This week, we learned the subjunctive – a grammatical term I’ve been fearing (without having any idea what it was) ever since I started.

It is easier than I feared, and it turns out, among other things, it is a way of telling people to do things in certain situations, and so as part of my practice, I’ve been forming sentences about Covid19 – if you are travelling on public transport, wear a mask – Si vous voyagez dans le métro, il faut portiez un masque.

It reminded me of earlier in the year when we learned the imperatif with a graphic of handwashing instructions.

 

Bit of beauty

I went back to the waratah bushwalk this week with geekinsydney, as I was astonished by the beauty of all the spring flowers (not just the waratahs). Here is a little one – less showy than the waratahs but worth a closer look.