Herd immunity as a process not target

My take on herd immunity in the UK:


Rather than focusing on whether we reached 70%-80% of the population – and the UK is not there yet – it is more useful to think of herd immunity as a process of virus suppression and elimination.

Herd immunity: can the UK get there?

My earlier articles on herd immunity:

Mutating coronavirus: reaching herd immunity just got harder, but there is still hope

Herd immunity: why the figure is always a bit vague

and some other recent articles:

We may never achieve long-term global herd immunity for COVID. But if we’re all vaccinated, we’ll be safe from the worst

What If We Never Reach Herd Immunity?


India and COVID

Statue of Buddha with lotos and birds

I love India. Thanks to my scientific collaboration (funded by the UK government and my New Professor Fund at Strathclyde) I have had the pleasure and privilege to visit the country twice and to meet fantastic Indian people. It is a country of huge potential, great challenges, landscape, and food like nowhere else. I was struck by the warmth of welcome and by the generosity of people.

The above picture, taken during my first visit in 2019, shows one aspect of India – the calm, serenity, and beauty of nature. But, I also like another version of that picture, taken in 2020:

Statue of Buddha with a female worker using a mobile phone

showing a connection between the past and the present – the bustle of a building site and ubiquitous mobile phones – the dynamics of the modern world and the advances in the technology of which India is very proud.

I was there last year when the pandemic was starting, flying out of India on 7th March 2020. There were people in masks at the airports and I carried a hand disinfectant. Over the next days, weeks and months, I was very concerned about the spread of the coronavirus in India, particularly with such challenges in living conditions, crowding, low hygiene, and deprivation. I was very unsure how it would be possible to control air- and droplet-borne virus in crowded cities, with streets as seen below:

A crowded street in India.

And yet, India largely managed to control the pandemic throughout the year 2020 and early 2021. Although there have been concerns about under-reporting, the outbreak until very recently was very far from a disaster that I had been fearing.

There have been many possible explanations for this relative success. The early reach of herd immunity was suggested, with serological data pointing to very high levels of antibodies in locations that had experienced early outbreaks, like Delhi. A very strict initial lockdown in March and April (shortly after I had left) seemed to contribute to the reduction in the potential of the virus to spread. Indeed Google mobility data showed a rapid, immense response to the lockdown.

Graph showing daily new confirmed cases per million, with a peak of abouy 70 pm in August 2020 and a rapid growth since March 2021

I have also read very interesting stories of self-help, community kitchens, mental health support, for example in Indian states like Kerala. As a result, India has so far been a COVID-19 success story. Unfortunately, not much longer.

Most likely it was the relaxation of rules in early 2021, seen in the mobility graph above that created a condition for the “second wave” which India is experiencing now. The raw numbers of new cases are now staggering – exceeding 300,000 new reported cases each day; there is almost certainly a huge under-reporting going on, so the true numbers are possibly 10 or more times higher – reaching perhaps millions. The scary part is the rapid, exponential growth, of cases and associated deaths (which also are most likely underestimated, possibly by at least a factor of 5), as seen below

Graph showing daily new confirmed deaths per million, with a peak of abouy 0.8 pm in August 2020 and a rapid growth since March 2021

Even more tragic is a constant stream of news about overcrowded hospitals, intensive care units running out of oxygen, and funeral pyres (cremation is part of traditional funeral rites). At the same time, vaccination progresses slowly (compared with e.g. the UK) despite India being one of the largest vaccine manufacturers.

Graph showing vaccination in India (10%) and the UK (50%)

The prognosis is, unfortunately, grim. There is some hope in the small drop in mobility (the relationship between mobility and infection is quite complex, but the higher the mobility the more chance for the virus to spread), but there appear to be only limited policy actions that could stop the spread.

Modelling of future scenarios is difficult, as there are plenty of unknowns:

(i) levels of pre-existent immunity and immunity acquired during the previous wave;

(ii) effects of non-pharmaceutical actions, like lockdowns and curfews;

(iii) huge heterogeneities in between cities and countryside, and poor and rich;

(iv) even the actual numbers are subject to huge under-reporting and so we do not know exactly the infection and death status.

With these caveats, some modelling results suggest that India is likely to experience a massive wave of infection, with the daily true number of cases reaching 15,000,000 (for comparison, London population is 9,000,000) and the daily number of deaths exceeding 13,000 (the Infection Fatality Rate, IFR, i.e. the ratio between the number of deaths and the number of infected individuals might be lower than in countries like the UK, as the Indian population is younger). The modelling results shown here come from an approach developed by the Institute for Health Metrics and Evaluation (IHME) which is an independent global health research centre at the University of Washington, USA.

I do really hope the scenario shown above will not materialise and somehow the epidemic will stop spreading through India. I am thinking of all my friends and colleagues in India, and all the people I met and seen, and I am praying that they are spared the infection and complications.

But I am also seeing the current developments in India as a warning to all of us. The scenario of a relatively small “first wave”, followed by relaxation of regulations too early and (relatively) open borders, combined with the rise of new strains (like B.1.617), is something that has been happening across the world and leading to a huge “second” or “third” wave. Many countries in Eastern Europe (like my native Poland), the UK, Chile, and others have shown a similar pattern, although not at such scale and rapidity.

The current events in India also seem to contradict many theories that have been used by lockdown sceptics and those who advocate the “light-touch” approach to COVID-19 – pre-existing immunity was supposed to protect the Indian population from the virus or herd immunity was supposed to have been reached a long time ago. Examples from Brazil, Chile and now India show exactly what might have happened in the UK, Poland, and across the world if the virus was allowed to spread uncontrolled. We might criticise our governments and scientists for many things, but not for imposing lockdown controls and developing and successfully administering the vaccine.

My thoughts are now with India. There is not much I can do to help with fighting the outbreak. But, I do hope that my work with colleagues in India would help long term in restoring the country’s food security post-pandemic. I look forward to future visits and tasting again the fabulous Indian food.

Has England reached herd immunity level – part 2

An update on the previous post on the UCL model and herd immunity levels. I just thought of adding a couple of further thoughts on this topic.

Firstly, there appears to be growing evidence of a potential for new strains (Brazil, South African, Indian) to overcome the immunity created by the previous infections.

The rapid growth of cases in Brazil and now in India suggests that either our estimates of Herd Immunity Threshold (HIT) are too low, or people who have SARS-CoV-2 antibodies can still be infected.

Secondly, there is an interesting interplay of heterogeneity and HIT. Heterogeneity, be it in the infectivity or susceptibility, is normally thought to lower the HIT. This basically is caused by the infection “burning” through the most susceptible parts of the population first.

As a result, we end up with a population where those who are not yet immune are protected by their lower susceptibility. This results in a lower value of HIT than predicted by the homogenous theory.

However, there is another trend that counteracts this process, and the one which seems more dominant at this moment. We now have a very uneven distribution of immunity due to vaccination.

Those who are potentially most likely to become infected (those in the BAME population, in deprived areas, or those who are vaccine-hesitant or anti-vaccine) also have the lowest vaccination coverage.

Thus, even if on average we can reach the HIT, these pockets of low-immunity, high-susceptibility/high-infectiousness, will keep the infection going and possibly creating the conditions where new strains can arise.

Even more arguments for the #ZeroCovid strategy… Let’s just keep going for a while longer, paying particular attention to those who are most at risk, to reduce the overall levels of infection until we can bring the vaccination higher.