coronavirus quarantine pandemic virus
(Image: AAP/James Ross)

Last week Sir Patrick Vallance, the UK’s chief scientific adviser, was publicly pilloried for seeming to suggest that the way to manage the coronavirus pandemic was to rely on herd immunity.

No matter that that’s not actually what he said, that’s what was heard. So what do we mean by herd immunity?

In brief, it means that if enough people are immune to a disease, then it becomes unlikely or unable to spread through the remaining vulnerable members of the community.

Sounds simple enough, and sometimes it is.

Take measles, for example. It’s a highly infectious disease that spreads very quickly amongst populations with no immunity.

As it’s such an infectious agent, to prevent measles spreading through a community requires rates of immunity of 90-95% (other less infectious agents, as coronavirus seems to be, may be contained with lower rates of herd immunity).

In modern society, most of that immunity comes from vaccination, rather than native disease. The effectiveness of herd immunity in measles relies on a couple of factors:

  • Immunity, whether from disease or vaccination, is usually lifelong. Antibody levels mostly stay high throughout life, conferring long term protection. 
  • The measles virus itself is stable — that is, it doesn’t mutate into different strains. Your immunity today is not suddenly going to become ineffective next week. Once a measles always a measles.

Sadly not all viruses are so well behaved, nor are our immune systems so efficient. Flu viruses for example obey neither of the measly rules: immunity from flu vaccination is often short-lived, sometimes only months, and not only are there multiple flu strains around, but they’re very good at mutating into different strains.

That’s why we have an annual flu vaccination program, and each year with different viral strains. 

As for COVID-19, it’s too early to tell. It’s very probable that, as with other coronaviruses, immunity, whether from infection or any future vaccine, will not be life long. Coronaviruses are generally also a wily bunch, pretty good at mutating.

What you see this year may not be what you get next. 

So I can’t see how herd immunity on its own is a viable option for “managing” COVID-19. A vaccine is at least a year away, so infection is currently the only way of developing immunity. 

And as we now know, infection means death for a certain percentage. Whether that percentage is three, one, 0.1 or lower, for a virus that is likely to infect tens, if not hundreds, of millions, that’s a lot of deaths.

That doesn’t make herd immunity irrelevant. As communities experience the virus and develop at least some level of immunity, it will eventually slow the spread. But no strategy that goes like this gets my vote:

  1. Nasty virus, help, what to do?
  2. Herd immunity is our saviour, let everyone get it as quickly as possible.
  3. OK, so quite a few (well, actually, quite a lot) will die, not to mention the health system being overwhelmed. Oh, and the morgues. But it’s mainly the old and sick, not me.
  4. Wait a long time, hopefully get a vaccine. Boost herd immunity.
  5. Hope immunity lasts/vaccine works/virus doesn’t mutate.
  6. Really hope this last bit is true.
  7. Problem solved.

Hence most countries have accepted that waiting is not a viable option, and that strong measures are needed  to reduce the rate of spread now.

This slowing will allow health services the time to develop capacity and deal with patients over a longer period, rather than being overwhelmed by a sudden and severe spike in demand.

Which in the end is pretty much what Sir Patrick and Boris ended up saying in their address about “herd immunity”. Bet they wish they’d said it up front.

Dr Nick Carr is a Melbourne-based GP, author and broadcaster. His interests include the care of the elderly, psychological medicine and paediatrics.