HERD IMMUNITY – A FALLACY, BUT... (Updated 11 August for CROSS-IMMUNITY)

The term “Herd Immunity” keeps cropping up.  People accuse the UK government of following a Herd Immunity strategy.

So what is Herd Immunity?  Is it relevant?

WHAT IS HERD IMMUNITY?

It's an unfortunate term likening people to herd animals.

It’s where 60-70% of a population acquire immunity to a disease, such that if someone is infectious, it becomes sufficiently unlikely they will encounter someone to infect that the disease tends to die out somewhat.

Herd immunity can be achieved by:

• Vaccines
• The illness itself

THE EFFECTIVENESS OF VACCINES

In the UK there is an annual programme of vaccination for older folk and certain others, but this is optional.  Most of the population do not get immunised.  So there isn’t herd immunity generally, though may reach that level in the group eligible for vaccination.

Nonetheless some 2,000 to 28,000 people have died each year from the complications of flu in recent years.

As noted in this introduction. persistent LongCOVID symptoms that effect all ages mean COVID-19 is a much more serious disease than flu.  Any immunity programme will need to cover vast swathes of the population of all ages.  A massive and expensive task, even if a vaccine is proven both safe and effective.

Obtaining the proof needed to vaccinate a large proportion of the population will take months. Initial indication of safety and efficacy by September, but we are talking well into next year earliest for mass vaccination.  Fingers crossed.

NATURAL HERD IMMUNITY

The Maths Has It

Simple maths suggests that it would take many years for infection to reach 60% of the population.

Taking England, with some 55 million people, that’s 33 million for 60%.  It would require some 90,000 new infections a day, with or without symptoms, to get to that level within a year.  At a more realistic 3000 a day, that’s 30 years.  Simply unrealistic.

Even that is assuming mild infections convey some immunity for a sufficient length of time.  Neither immunity nor duration are yet proven.

So it was especially concerning that Sir Patrick Vallance, the UK’s Chief Scientific Advisor talked seriously about the UK aiming for herd immunity.  This was back in early March, 10 days before the UK’s national lockdown  Key part is from 4min35, with the previous section setting the scene.  As a result of a vociferous backlash, the UK government had to quickly assert it was not following a Herd Immunity strategy.  Yet the suspicion has never totally disappeared.

BUT WHAT ABOUT PARTIAL HERD IMMUNITY?

Sweden has followed a different strategy from the rest of Europe.  Sweden has chosen to adopt a voluntary lockdown , which they believe to be nearly as effective as legal lockdowns, for two reasons:

• In acknowledgement that COVID-19 is a long term disease requiring public sacrifice for long periods of time.  More sustainable to explain the reasoning, and get people to adhere to advice voluntarily.  For example travel has reduced as much as in neighbouring countries
• To minimise the ‘side effects’ of a lockdown.  Schools and businesses have been allowed to stay open, with the exception of gatherings over 50 people and a few other restrictions

They’ve been able to stick with this strategy by keeping within their health service capacity, and explaining to the Swedish population that a higher death rate in the short term is to be expected, but will not appear so high as time goes by.

They insist this isn’t a Herd Immunity strategy, but have found a immunity bonus according to their chief epidemiologist, Dr Anders Tegnell:

• Nobody tested positive for COVID-19 has had a recurrence (at 20min25s).  That suggests a level of immunity, but not yet clear for how long.  Six months is one estimate
• Total cumulative cases, including where mild or no symptoms, could be as high as 20% or more in some places in Sweden (at 21min15 to 22m40), which confer immunity.  Though other areas appear very low. 
• The levels seem to be enough on average across the country to noticeably reduce R, and make it easier to defeat some local surges

Update 11/8/20: But see notes about cross-immunity below for possible explanation of  high levels of immunity.

Sweden has been heavily criticised for higher death rates than most other Scandinavian and European countries, in the short term.   Although lower than some other countries like UK, that Dr Tegnell believes are inherently more comparable.  We’re seeing enforced lockdowns being released across Europe, and countries such as Spain now suffering a massive resurgence, Sweden’s “voluntary” policy looks as if it is already being better than formal lockdowns.

As we know that COVID-19 is likely to be more prolific in winter than summer, due to people being indoors more, the time to assess Sweden’s strategy will be each March, 2021 and 2022.  By then they may have been overtaken in deaths by other European countries, yet have kept schools and businesses open.  Only time will tell.

It will then be interesting to assess how much partial Herd Immunity will have helped them.


CROSS-IMMUNITY  (UPDATE 11 AUGUST 2020)

Evidence is mounting of cross-immunity , where it is hypothesised that immunity to COVID-19 has been achieved for those people who have had a coronavirus type of common cold.  The four such coronaviruses account for 15% or more of the incidence of common cold infections .  Cross-immunity would create higher levels of immunity to SARS-COV-2 than from it alone.


T-cells are an important part of the human immune response to infection.  Levels of T-cells that provide immunity respond to SARS-COV-2 are being found in around 50% of city populations and elsewhere where common colds are common.  T-cells are kept (or are continually produced) for many years.  They've been found in survivors from the original SARS 17 years later.  Though T-cells tend to reduce in number in older people,  That may explain why oldies are been more likely to catch COVID-19.

Antibody testing has identified around 70% of people in the UK having had COVID-19 without symptoms.  Or are these tests just finding antibodies left from the corona-based common colds/?  I suspect they are, as it seems possible that the 'soup' of IgG antibodies for each coronavirus is so similar as to overlap and thereby be indistinguishable to a test. 

This new study looking at T cells, though not yet peer-reviewed, found:

• For people who have been confirmed as having had COVID-19; "Circulating SARS-CoV-2-specific CD8⁺ and CD4⁺ T cells were identified in ∼70% and 100% of COVID-19 convalescent patients, respectively"
•  For those who weren't confirmed as having had COVID-19: "...we detected SARS-CoV-2-reactive CD4⁺ T cells in ∼40%–60% of unexposed individuals, suggesting cross-reactive T cell recognition between circulating “common cold” coronaviruses and SARS-CoV-2."

In the latter case it is unclear whether those individuals could have had COVID-19 asymptomatically.  But the chances are these ae people who hadn't had it

So if around 50% of an area's population have had a coronavirus common cold, only 50% would be susceptible to COVID-19.  Then the 60-70% target for herd immunity to COVID-19 is reduced substantially.  Estimates are it would only need to take around 20-30% with COVID-19 itself..

That's the sort of level being found in Stockholm, Sweden for example, as noted above.  That would explain why Sweden's new cases have reduced so quickly.  Dr Tegnell was saying recently that he didn't understand why this happened, implying he wasn't relying on cross-immunity for the country's strategy.  So it looks like Sweden has struck lucky!


WHAT DOES THAT MEAN FOR EACH COUNTRY'S STRATEGY?

It is perhaps too early to rely on cross-immunity.    But it is certainly looking like the herd immunity concept might apply to coronaviruses as a set, rather than COVID-19 alone.

Hopefully we'll see infection rates dropping worldwide as that type of herd immunity takes effect for COVID-19.

If this happens in England, hopefully we can reduce or eliminate the need for local or national lockdowns.  But for now the experience in the north west, where infection levels remain stubbornly high, suggests we're not in that position yet.