Topic 10.4

Sometimes, we notice that half of the people who get seriously sick with COVID-19 are vaccinated, and the other half aren’t.

Does this mean that vaccination is not working?

To answer this question, you need some facts:

• When Australia went through the COVID-19 pandemic, the original aim was to get 80% of people vaccinated.
• Let’s say the vaccine was 75% effective in protecting against serious diseases (hospitalisation and death).

Let’s look at the numbers and proportions:

• Take 1,000 people, of which 80% are vaccinated – 800 are vaccinated, but 200 are not.
• Of the 800, 75% are vaccine-protected, so 600 can’t catch the disease, but 200 can.
• Of the 200 unvaccinated people, none are protected, so 200 can catch the disease.

If there was a drastic exposure week where everyone was exposed to the infection, you could see 200 vaccinated people with the disease and 200 unvaccinated people with the disease.

It could look like vaccination wasn’t working – with the same number of cases in each group.

But the vaccine is working! It is 75% effective and stops 3 out of 4 infections.

When a lot of people are vaccinated, like 80%, sometimes it looks like the same number from each group gets sick.

Here’s another example for you to consider, where hospitalisations due to COVID-19 may appear to be more common in the vaccinated. Why is that not the case?

Cholera is an acute disease caused by the bacterium Vibrio cholerae.  It gives people bad diarrhoea, bad enough to sometimes kill. A person can become infected by food or water contaminated with human waste. The disease is common in parts of the world with poor water quality and can cause severe dehydration from diarrhoea and vomiting.

There are vaccines available these days to protect against it. This wasn’t always the case.

In 1854, there was an outbreak of cholera in Soho, London. A man named John Snow investigated the cases and concluded that the water pump in Broad Street was the source of the cholera.

He also investigated the two water supply companies at the time. The Lambeth company took water from a clean part of the Thames River. The Southwark and Vauxhall company took water from more polluted parts of the river.

.John Snow mapped the disease Cholera. 

Let’s say the Southwark & Vauxhall company provided water to pumps servicing 40,046 households, of which 286 reported a cholera death (i.e. the incidence of cholera deaths in this group of households was 286 out of 40,046).

The Lambeth company supplied water to pumps servicing 26,107 households, of which 14 reported a cholera death (i.e. the incidence of cholera deaths in this group was 14 out of 26,107).

We know the households receiving water via the Southward & Vauxhall company were getting water from the polluted areas of the Thames River, so we’ll call them “exposed” to the pollution.

The households receiving water from the Lambeth company were receiving cleaner water, so we’ll call them “unexposed” to the polluted water.

If we put all this into a table:

Epidemiologists call this a 2 by 2 table.

Relative Risk = Incidence in exposed ÷ Incidence in unexposed

The relative risk of cholera deaths in households supplied by Southwark & Vauxhall (compared to households supplied by Lambeth)

= (286÷40,046) ÷ (14÷26,107)

= 13.3

That is, people in households with water supplied by Southwark & Vauxhall were about 13 times more likely to die of cholera than people in households supplied by Lambeth.