Explainer: Why no vaccine can ever be 100% effective

The world eagerly welcomed the news of the three new coronavirus vaccines, in particular their efficacy.

Preliminary results of clinical trials indicate their efficacy ranging from 62 percent to 95 percent.

None of them recorded 100% and that could sound disconcerting. But the reality is no vaccine does that. Indeed the latest coronavirus jabs may provide better safety than many other vaccines, experts say.

"Seventy or 90 per cent effectiveness is remarkably high," Dr Andrew Preston, a reader in microbial pathogenesis at the University of Bath, told The Daily Mail. "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 vaccine offer full protection, and does it matter?

Vaccines function by developing a 'memory' of the disease once the immune system comes into contact with it again—it identifies the disease and causes a 'bigger and longer-lasting response' which means that it has little chance of taking hold and causing symptoms, explains Will Irving, professor of virology at the University of Nottingham.

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, 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."

Although the goal is a 100% effective vaccine, it is difficult to attain because human make-up is so different from one another.

The three recently reported coronavirus vaccines with efficacy rates of up to 95 per cent are actually high relative to other vaccines; for example, flu jab is only 50 per cent effective on average.

Compared to two doses of measles, mumps and rubella (MMR) jab, 99 per cent of people will be safe from measles and rubella and 88 per cent from mumps; while whooping cough vaccine is initially 80 per cent successful, safety declines to 60 per cent after four years, the Canadian Medical Association Journal reported 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 is not about preventing symptoms in the individual—it may also stop the disease from circulating.

For example, if more than 90% of citizens are vaccinated with MMR jab, this decreases the amount of circulating disease and thus prevents those that are not vaccinated—so-called herd immunity. In the end, this will lead to the eradication of a disease, as with smallpox (the jab was 95 per cent effective).

How many people need a Covid-19 vaccine to establish herd immunity is unclear; it depends on how contagious the disease is—the 'R' or reproductive rate—and how successful 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.