Herd Immunity and Legionnaires’ disease

Herd Immunity and Legionnaires’ disease

What is Herd immunity?

Herd immunity has been a bit of a catch phrase during the ongoing pandemic. It sounds reassuring that somehow the ‘herd’ is protected. The general assumption seems to be that once 50- 60% of the population have been infected and are then immune then we will have control.

But if we dig a little deeper into what it means we’ll see that the 50% idea is an urban myth. We’ll also see that ‘herd immunity’ is not possible for many diseases – including Legionnaire’s disease.

So what is Herd immunity?

Well for starters, it only applies to diseases that you can catch from someone else (transmissible) – so that rules Legionnaire’s disease out!

To build on that, it is based on several different factors that are unique to different situations. In fact, there is a calculation that can be used to work out the percentage of population that need to be immune for the effect to work.

The simple version of this is (R0 − 1) divided by R0 .

R0 is the number of infections likely to be passed on by a single case. This number then calculates what proportion of the population need to be immune to stop infections being passed on by a single case.

This is different for different diseases. The idea of herd immunity is to have a percentage of the population immune (either by vaccination or controlled exposure) so the disease can no longer ‘move around’.

Example: if an infected person is likely to infect 4 others. Then R0 is 4.

so R0-1 = 3 divided by 4 = 0.75.

As a result it means 75% of the population need to be immune for herd immunity to work.

Herd Immunity Factors

Population density – pretty obvious really. If you live near lots of people you’re more likely to get disease from them and pass it on.
Susceptible population – for any disease there will be susceptible groups – for example adult meningococcal disease is pretty rare.
Infectivity – some diseases are very easily passed on. A single exposure might be enough – or it might take a few. In the case of Legionnaires’ disease no amount of exposure will pass it on from an infected person to anyone else.
Transmissibility – if the disease needs a special way to infect you (eg Hepatitis B from blood contact) then that limits its ability to move around. But, if a disease like COVID -19 can move around by droplets, aerosol, surface contamination then it changes the game.

If the factors above get put together in a formula (in a more complex version of the formula above!) then it is clear that for each disease the requirements for Herd Immunity are different.

So, as examples – to achieve herd immunity for Diphtheria it requires around 70% of the population to be immune (and it’s not very catching); for chicken pox it needs around 90% to be immune (very catching).

There’s still not enough data to work out exactly what will be required for COVID-19 – but it looks like 60% might be enough.

Herd Immunity and Legionnaires’ disease

It is also clear that, sadly, for some diseases it is unachievable. Legionnaires’ disease is a prime example of this. The disease is not passed on from person to person, but by contaminated water.

A vaccine is not available, probably for two good reasons.

1) Only a small percentage of the population get the disease. This means preparing a vaccine for a small number of individuals would be very expensive – and not economically viable.

2) The population that does get Legionnaires’ disease is mostly people with poor or weakened immune systems. This means the vaccine might well not ‘take’ because the persons immune system cannot recognise it. Worse still, depending on the type of vaccine, it could cause disease rather than prevent it!

So Legionnaires' disease and other non-transmissible diseases do not fit into this formula. We understand quite well the ‘susceptible population’ and ‘transmission mode’ – but the rest of the data we need to complete the formula is unknown.

If Herd Immunity can’t happen…what do we do?

Well, pretty simply – we do exactly what we’ve been doing in the current pandemic - preventing. Working out ways to stop the disease moving around is our best option. Diseases that are not vaccine preventable but are high risk need to be managed. This is where the prevention and risk management strategy comes in to play.

The old adage ‘prevention is better then cure’ is attributed to the philosopher Erasmus (pic. below) in the 16th century – but it is an enduring fact in public health.

Here in 2020, the same message is being promoted with regards to Legionnaires’ disease during the COVID-19 pandemic.

Several independent organisations (see Further Reading below) have now issued warning relating to elevated risks of Legionella infections during the pandemic.

They have also noted that the highest risk population for COVID-19 is pretty much identical to that for Legionnaires’ disease. This increases the risks of a potentially lethal co-infection.

Legionella Risk Management Plans and Water Safety Plans

If you have a Legionella risk management plan in your health / aged care facility now is a good time to check that it is current and active. A risk management plan that is not active and current is of no value to you. Plans should be reviewed annually and checked to make sure they are current and being implemented.