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.


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

Some momentous developments

A report came out recently suggesting that there is a large variability in the levels of antibodies to COVID-19 and that they tend to disappear in a matter of days and weeks. This has been reported in this preprint and that preprint so it has not gone through all checks. But it agrees with some other evidence, discussed here, that patients can indeed get the virus twice. There are now more studies pointing in that direction, and indeed, the similarity of SARS-CoV-2 virus to other coronaviruses would suggest that the immunity might not be long-lasting.

If this is true, it has some very serious consequences. It would mean that the whole idea of “herd immunity” is not valid in the long run. It would mean a collapse of the Swedish strategy which also – despite all contradictions – appears to be the British way of dealing with the virus. If indeed SARS-CoV-2 is more similar to a common cold than to flu, we do indeed need to learn to live with the virus for a long time.

On the positive side, a serology study which looked at both exposed and unexposed individuals found a large variety of immunological responses. If I understand this study correctly (and I am not an expert in immunology), many individuals who have never been exposed to SARS-CoV-2 are showing an immunological response. In fact, they claim 81% do.

Again, if this is true, it suggests three things. Firstly, the tests are likely to show a lot of false positives. In other words, if somebody is tested and gets a positive result, it does not mean this person was exposed to SARS-CoV-2.

Secondly, and related to my point about the short-lasting immunity, even if a person was indeed infected, there is no guarantee they are immune. Thus, any idea of “immunity passports” is irrelevant.

But thirdly, perhaps there is actually a high – although not as high as 80% – the general level of resistance to the virus. Perhaps the epidemic will indeed slow down soon and the virus will become a nuisance (like a common cold) rather than a deadly enemy.

In the light of these findings, I marvel about the complexity of creation – and about the peculiar beauty of viruses and the ways our bodies deal with them.

But also a word of warning. As I said in my article in The Conversation about the herd immunity, politicians often get fixated onto simple concepts – like “herd immunity” – which are in fact not simple at all.

24 weeks

I have been reminiscing about the pandemic today and looking at the calendar. Just over 24 weeks ago (31st January), the first UK case was detected, followed 4 weeks later by the first COVID-19 case in Scotland (1st March) – 20 weeks tomorrow.

My thoughts were drawn to this early days by the article in Financial Times, titled Inside Westminster’s coronavirus blame game, published on 16th June. It is a very interesting piece, not only from the political of view, but also because it discusses in great detail the thinking behind the decisions taken at that time. It talks primarily about the critical week of 16-23rd March, just before the lockdown, but when the UK government was apparently abandoning all testing and appearing to do very little.

But things were already getting out of the government hands. My last day an the uni was 10th March, as I was working at home on the 11th and attended a conference in Edinburgh on the 12th. University closed down on the 16th and the full lockdown started on the 23rd. There were nearly 13 thousand cases reported in the UK by then. Nearly 300 thousand have been reported since.

I noted that my first blog on coronavirus appeared on 12th March and included these lines:

You need to understand that a politician needs to carefully balance the pros and cons of any action they take, as the consequences might be massive. This means that they tend to either do nothing (President Trump before the 11th of March) or go into a full action (President Trump after the 11th of March). They will not want to be accused of needlessly spending money. Still, on the other hand, they do not want to see TV programmes about hundreds of people dying in hospitals. In fact, our own research shows that for people, there are two rational strategies, do nothing, or act with a full force. Economists call it a ‘bang-bang’ approach.  So, expect the governments to swing between different options as they face the biggest crisis since 2008.


With a bit of a Schadenfreude, I note that the next 20 weeks have illustrated very well the approach to coronavirus, not only of the UK government but also worldwide. Even now, the UK government is encouraging a full return to work, as if the virus has already gone having changed the strategy from a deep lockdown to a full “normality”.

Do I still think the early lockdown was the best thing to do in March? I probably still do think that the government should have acted sooner and stronger, and I regret not having spoken then, not having written to the MP, or to the press. It probably would not have made any difference (I am not part of SAGE, or any similar advisory body), but I would have had a satisfaction of having done something right.

What I certainly did not expect then was how well the UK population would obey the lockdown once it was announced. Google mobility data show that the UK had one of the deepest reductions in mobility, although there is some evidence that the local scale (village, street) mattered more than the country level. Perhaps an early lockdown could have arrested the disease spread better than the late one (as in New Zealand, or South Korea), would have lasted shorter and costed less.

Or, perhaps, the Swedish model, with a partial lockdown and reliance on the voluntary social distancing, but better done, with a much higher protection of the vulnerable population (elderly, care homes, immigrants), would have been a better solution.

Mathematical epidemiologists really need to have a serious discussion with economists, something that has not been done too well over the last months. This will allow not only evaluation of what could have been done, but – more importantly – how to deal with the ‘winter wave’ if or when it comes, or with the next big pandemic, when (not if) it comes.

The article in Financial Times makes it clear that one of the problems faced by the government and SAGE in March was that they were all expecting a different epidemic. The next time, we need to be better prepared.