Why no vaccine can ever be 100% effective: Wide variation in the population’s immune systems mean guaranteed protection is impossible

The announcement of not one but three coronavirus vaccines has been widely welcomed, in particular, their effectiveness.

Preliminary results of clinical trials into the three jabs show their effectiveness — how good they are at stopping people from getting Covid symptoms — varies from 62 per cent to 95 per cent. 

The news that none of them reported 100 per cent might sound perplexing. But, the fact is, no vaccine does. Indeed, experts believe the new coronavirus jabs may offer better protection than many other vaccines.

The three coronavirus vaccines that have recently reported results with effectiveness rates of up to 95 per cent are actually high compared with some vaccines; the flu jab, for example, is only 50 per cent effective on average [File photo]

‘Seventy or 90 per cent effectiveness is remarkably high,’ says Dr Andrew Preston, a reader in microbial pathogenesis at the University of Bath. 

‘It means nine out of ten vaccinated people would not become ill with Covid-19 if they were exposed to it.

‘The vaccine may not stop the person catching the infection, but it would stop them getting symptoms of the disease if they caught it.’ But why can’t a jab offer full protection, and does it matter?

Vaccines work by creating a ‘memory’ of the disease should the immune system come into contact with it again — it recognises the disease and triggers a ‘bigger and longer-lasting response’, which means it doesn’t have a chance to take hold and cause symptoms, explains Will Irving, a professor of virology at the University of Nottingham.

‘When developing any vaccine, clinical trials are carried out to find out whether it is effective and if it is, how good it is,’ he explains. 

‘To do this you have a group of people that’s vaccinated and another that isn’t, the placebo group. You then count the number of cases of the disease in each group over a period. If you have 25 cases of disease in the vaccinated group and 50 cases in the placebo, the vaccine is 50 per cent effective — i.e. it has prevented half the people in the vaccinated group from getting the disease.’

Preliminary results of clinical trials into the three jabs show their effectiveness — how good they are at stopping people from getting Covid symptoms — varies from 62 per cent to 95 per cent [File photo]

However, while a 100 per cent effective vaccine is the goal, it is impossible to achieve because human make-up differs so much.

The three coronavirus vaccines that have recently reported results with effectiveness rates of up to 95 per cent are actually high compared with some vaccines; the flu jab, for example, is only 50 per cent effective on average.

In comparison two doses of the measles, mumps and rubella (MMR) jab will protect 99 per cent of people from measles and rubella and 88 per cent against mumps; while the whooping cough vaccine is initially 80 per cent effective, protection drops to 60 per cent after four years, reported the Canadian Medical Association Journal in 2016 — which is why children have booster jabs.

‘There is too much variation in the population’s immune systems for any vaccine to be 100 per cent effective,’ says Dr Preston. ‘For example, as we age, our immune system responds less well, which means vaccinating older people in general is a problem.

‘That’s why the flu and pneumonia vaccines, which are largely aimed at older people, contain adjuvants — compounds that boost the immune response to the vaccine, meaning we are more likely to produce antibodies and be protected.’ 

Any condition that weakens the immune system, including obesity, may also affect our response to a vaccine.

‘Obesity creates an inflammatory state in the body and it’s thought the heightened inflammatory state may exhaust the immune system, making it less able to respond to vaccines,’ explains Dr Preston.

Vaccination won’t just stop symptoms in the individual — it can also stop the disease circulating. 

For example, if more than 90 per cent of people are vaccinated with the MMR jab, this reduces the amount of circulating disease, also protecting those who are not vaccinated — so-called herd immunity. Ultimately, this can lead to a disease being eradicated, as happened with smallpox (the jab was 95 per cent effective).

Exactly how many people need to have a Covid vaccine to create herd immunity is unclear; it depends on how infectious the disease is — the ‘R’, or reproduction rate — and how effective the vaccine is.

‘If measles vaccination rates drop below 90 per cent, for ex-ample, there are outbreaks of the disease because measles is fantastically infectious, with an R of around 15,’ says Professor Irving, explaining that SARS-coV-2 (the virus that causes Covid infections) has an R rate of up to 3.

It’s also not clear whether the one in ten who are vaccinated against Covid-19 and yet to show symptoms would be more severely ill if they hadn’t had the vaccine.

Another unknown is whether being vaccinated will stop people from being infectious, so called sterilising immunity.

‘All vaccines are designed to stop the disease they are targeted against, but it is very difficult to generate immunity that actually stops infection,’ says Dr Preston.

‘So no matter how many people are vaccinated, the virus will still circulate. This is the case with the whooping cough vaccine.

‘It stops the Bordetella pertussis bacterium infecting the lungs, which causes this dangerous disease, but it doesn’t stop people from becoming infected in their upper airways, and these people can still transmit the infection.’

Until we have more answers, science has to be transparent, says Dr Preston. ‘No medicine is without risk and we need to be honest about that while at the same time fighting disinformation,’ he says.

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