How close to herd immunity are different parts of the world? Reflections #33

Insight

As countries start to measure how many people have had Covid19 and are (hopefully) immune, we are still a long way from herd immunity. Even in New York (which has the highest level of past infections at around 25% of the population), a second and third wave the same size as the first would be needed before there was herd immunity there.

A number of countries have started to test for community prevalence of Covid19. That gives us more insight into how many people haven’t yet been infected, and therefore how far away we are from herd immunity.

What is herd immunity? Here is a good summary of what it means.

When most of a population is immune to an infectious disease, this provides indirect protection—or herd immunity (also called herd protection)—to those who are not immune to the disease.

For example, if 80% of a population is immune to a virus, four out of every five people who encounter someone with the disease won’t get sick (and won’t spread the disease any further). In this way, the spread of infectious diseases is kept under control. Depending how contagious an infection is, usually 70% to 90% of a population needs immunity to achieve herd immunity.

Basically, the level of immunity to a disease required for “herd immunity” depends on how infectious the disease is. If people are going about their normal daily lives, then to avoid outbreaks, infectious people would rarely come in contact with people who aren’t immune, so the disease doesn’t spread further. One of the most infectious diseases is measles, which needs an immunity level in the community of 92-94% for herd immunity.

For Covid19, the general consensus is that without social distancing, herd immunity to Covid 19 will be achieved with around 60-70% of the population immune from the disease.  That means that people in the community who aren’t immune are protected from the virus by people who are immune not spreading it. Right now, instead of that approach, we are protecting all of us who aren’t immune by keeping physically away from each other, so that we can’t spread it even without immunity.

So I thought it would be good to look at the big outbreaks around the world, to see if any of them have got close to herd immunity. And there isn’t anywhere that seems to have got close.

In Spain, overall 5% of Spaniards have contracted the virus (based on antibody testing), with Madrid having more than twice that level, at 11.3%, according to this article.

In France, around 4.4% of the total population will have been infected by mid May, with around 10% infected in Ile-de-France (the region that includes Paris) according to this study. The study analyses hospitalisation and fatality data, compares it with the experience of the Diamond Princess. This is consistent with with this analysis of blood donors in April which concludes 3% of the population in the French Oise region had been exposed.

In Italy, this study estimated a prevalence of 4% of Covid19 across Italy as a whole, with the highest proportion infected in Lombardy of 13.3%. The study worked backwards from the numbers of deaths to determine infection rates.

In an investigation of one town in Germany, around 14% of the population had antibodies to Covid19 in early April.

And as I’ve blogged before, there have now been a few studies in the US, summarised in this article, with the highest proportion of the population infected being nearly 25% of New York City.

In the UK, the Office of National Statistics did a pilot survey of 10,000 people (excluding care homes) and found 33 positive tests – suggesting only 0.27% of the English population had Covid19 during the survey. Interestingly, there was no difference by age (including those under 20). This study didn’t find people who have had Covid19 and recovered, so will automatically be lower than the earlier prevalence studies.

Sweden released a similar survey to the UK suggesting that at the time of the survey (late April) 0.9% of the population had an ongoing infection with the population of Stockholm having a 2.3% infection rate.

All the data I have found above suggests that even in the worst hit communities around the world, we are a long way off herd immunity. In New York, for example, the number of infections (and therefore deaths, with no vaccine) would need to triple before herd immunity was achieved. New York City has had around 20,000 (officially) or 25,000 (including excess deaths) deaths to date, so achieving herd immunity, even in a place as hard hit as New York, without a vaccine or good treatment, would lead to another 50,000 or so deaths in New York alone.

We’ll be staying in a socially distanced world for a while yet, unless we get to full eradication.

Link

This article is another good summary of what we know about how this virus spreads.  It has a formula:

Successful Infection = Exposure to Virus x Time

and a reminder that while it is possible to be infected from an infectious person breathing on you, the biggest outbreaks have come from places where there is close contact, animated conversation, even shouting, and a very closed environment. One of the biggest outbreaks used as an example is a choir from Washington State in the US:

Choir: The community choir in Washington State. Even though people were aware of the virus and took steps to minimize transfer; e.g. they avoided the usual handshakes and hugs hello, people also brought their own music to avoid sharing, and socially distanced themselves during practice. They even went to the lengths to tell choir members prior to practice that anyone experiencing symptoms should stay home. A single asymptomatic carrier infected most of the people in attendance. The choir sang for 2 1/2 hours, inside an enclosed rehearsal hall which was roughly the size of a volleyball court.

Singing, to a greater degree than talking, aerosolizes respiratory droplets extraordinarily well. Deep-breathing while singing facilitated those respiratory droplets getting deep into the lungs. Two and half hours of exposure ensured that people were exposed to enough virus over a long enough period of time for infection to take place. Over a period of 4 days, 45 of the 60 choir members developed symptoms, 2 died. The youngest infected was 31, but they averaged 67 years old. (link)

Sadly for me, choir meetings seem right up there with major spectator sport events, and religious events, as something that will be one of the last to reopen.

Following on from previous post of the control cycle, New York State has published a set of conditions for reopening in stages:

The loosening of restrictions in New York will be considered on a regional basis, based on the following criteria. These criteria are designed to allow phased reopenings to begin in each region only if:

• The infection rate is sufficiently low;
• The health care system has the capacity to absorb a potential resurgence in new cases;
• Diagnostic testing capacity is sufficiently high to detect and isolate new cases; and
• Robust contact-tracing capacity is in place to help prevent the spread of the virus.

Each one of these criteria comes with specific metrics, and ways of testing whether they are holding true. I hope that we have this in Australia, too, although I haven’t seen it – maybe it is behind the scenes.

Life Glimpses

The school student in our house went back to school this week – so far the social distancing there appears to be working in the classroom, but less so in the playground. It must be very hard, though, even with only 25% of them there at any one time. Human nature when you see your friends is to stand and chat.

I visited a big medical clinic for a regular checkup this week, and had my first taste of proper Covid19 screening. My temperature was checked before I was allowed in the door, plus I had to read and agree to a long set of statements of all the ways I was feeling well before I was allowed properly inside to take some of the copious free hand sanitiser on offer. Once I had passed, I was given a little coloured sticker to denote my status, which explained something I had been wondering about the daily NSW press conferences – I kept thinking their coloured sticker was some commemoration, but I suspect it is the same thing – you get a sticker every day to prove you are allowed in.

And this week a colleague offered to meet with me face to face, now that we are opening up again. I don’t think I’m quite ready for that yet, (we could even have caught up for coffee!) despite the much lower number of cases in NSW this week, but it did make me wonder what my own personal metric should be for in person meetings.

Bit of Beauty

In getting to the medical clinic this week, I decided to avoid public transport and ride my bike. So on the way home, I had a quick stop at Observatory Hill to take this picture. It doesn’t fully capture the glory of the morning, but I was still quite pleased with the picture.