Herd immunity and future scenarios

Last week, the media picked up a study from Mumbai, India, where it was reported that 57% of people who had been tested showed exposure to the virus. While it is not completely clear what this means for immunity (the survey looked at antibodies in the blood; see vox.com for a good explanation of what this means) and particularly long-term immunity, it has raised questions about whether we might be nearing “herd immunity” and therefore the magic point when the disease will stop spreading on its own.

Additionally, they reported a relatively low death rate (Infection-Fatality Rate, IFR) – the data seem to suggest that many people are getting infected, but only a few die.

https://indianexpress.com/article/cities/mumbai/in-mumbais-slums-simple-act-of-bathing-and-relieving-oneself-becomes-more-daunting-for-women-6342816/

Before we start jumping to conclusions, there are few words of caution. I have tried to get hold of the details of the report, but a search on medRxiv, the usual place where scientists report their findings, but could not yet find it. We are therefore basing any speculations on newspaper articles. Secondly, the 57% figure was only for Mumbai slums where presumably the transmission was particularly high; for other localities, the number was much lower, perhaps around 16%. This is more in line with places that experienced high coronavirus numbers, like New York (ca 24%).

Thirdly, as mentioned in my The Conversation article a couple of weeks ago, there is still a lot of uncertainty on what exactly these studies tell us about long-term immunity to SARS-CoV-2. Is it for life, like for measles? Or, for a year, like for the flu? Or, weeks, like for the common cold? And, what is the current outlook for the COVID-19 pandemic?

Almost 19 weeks we published the article in The Conversation outlining different scenarios for the pandemic; followed up 10 weeks later with another one on the second wave. How did the outlook change since then?

One of the scenarios that we listed 19 weeks ago was of a large first wave followed by slow eradication:

https://theconversation.com/four-graphs-that-show-how-the-coronavirus-pandemic-could-now-unfold-133979 25th March 2020

While it is a gross oversimplification of what is happening, I still believe this is a correct picture in the long term, although the time scale will be much longer. The “true” graph arguably will be very jagged, with ups and downs of local epidemics, and the peak will last longer than about 30 weeks (a bit like the picture below from the June article), but there will be no “silver bullet” solution that will stop the disease quickly and painlessly.

https://theconversation.com/coronavirus-what-a-second-wave-might-look-like-138980 1st June 2020

Firstly, based on what we currently know, I have strong doubts about the epidemic “burning out” by itself through reaching the “herd immunity” globally – although we might reach “herd immunity” in some confined places. We simply do not know at this stage whether the immunity is lasting and I personally have doubts about it.

Wide availability of the vaccine might change the picture, but again it is not clear to me that it will confer solid and long-lasting immunity. Repeated vaccination (boosters) might be needed to keep the immunity, and it will take a long time to bring the disease worldwide to sufficiently low levels. It has taken 200 years to eradicate smallpox; rinderpest and polio eradication was quicker, but still took many years and a concerted international effort (polio is not yet fully eradicated); measles was never eradicated worldwide, but almost eradicated in Europe and the US and making come back due to decreasing vaccination levels . In my opinion, even with a working vaccine we will have to live with the virus for a long time.

And then, there are likely to be other viruses ready to jump species, unless we become much more cautious.

So, what are the lessons? Firstly, we need to plan long and cautiously. It is clear to me that currently many governments (and people) apply a strategy of “pendulum swings”. We have actually seen this type of strategy emerging in individual people’s behaviour when we challenged them with a simulated epidemic (our 2015 BMC Public Health paper). In this approach, people slam the epidemic when it is threatening, but relax the control measures as soon as it recedes, even if it is not completely gone. As a result, the epidemics come and go in a long-lasting cycle, while the effective reproductive number is close to 1, while the economic losses mount with each cycle of the lockdown. The other problem with this approach is that with each cycle people become more tired of regulations and less likely to obey the rules, as we already see in the crowded beaches and pubs.

I still believe that we should have slammed the virus much stronger and much earlier than what happened in the UK (and other countries). But I now think that this should have now been followed with a much more controlled strategy than reopening everything.

In the long run, we will need to learn to live with the virus. Travel will become much more difficult, with more people having to spend holidays near home; we might need coronavirus vaccination before we travel to places where coronavirus is still present (like polio or typhus).

I think what I am getting at is that we should perhaps look at the virus as an opportunity to change things for better, as a challenge to adapt to the new situation, not desperately hoping to get back to “normality”.

The world at the end of 2020 will be very different to the one a year ago. But these speculations take us too far from the main topic of this blog.


It is only after having written this post I learned that the WHO official has recently used the term “silvery bullet” in the context of the coronavirus pandemic. He said:

“However, there is no silver bullet at the moment and there might never be.”

“It’s completely understandable that people want to get on with their lives, but we will not be going back to the old normal,”

WHO Director-General Tedros Adhanom Ghebreyesus