Being part of the herd saves lives

13 April 2015
By Rebecca Gladstone

Barbara Bellingham, Wellcome Images

Vaccinating the population saves lives.
Credit: Barbara Bellingham, Wellcome Images

Streptococcus pneumoniae AKA the pneumococcus is a notorious bacteria that can cause countless types of infections from rapidly fatal meningitis and septicaemia, to pneumonia and common ear infections. The pneumococcus is in fact the leading cause of child death worldwide killing up to 1,000,000 children each year.

A number of new pneumococcal vaccines have been licensed to protect children against pneumococcal infections, which are now being introduced around the world. However, the full potential of these life-saving vaccines could be wasted if we fail to vaccinate enough people.

What’s getting up your nose?

Pneumococci can be found harmlessly living in the noses of around 30 per cent of healthy kids in the UK without causing an infection. This colonisation is not completely benign, though, as it is a way of hitchhiking through the population from person to person, resulting in new infections in the most at risk.

Crucially, a pneumococcal vaccine not only protects the individual against infection by readying the immune system but also prevents colonisation of their noses and subsequent spread of the pneumococcus through the healthy population.

Vaccination needs you and your kids

The reduction in spread between vaccinated individuals also means that infections in unvaccinated individuals can happen less often. This phenomenon is called herd protection.

There is a major caveat, though; enough children need to be vaccinated so that there are not enough unvaccinated individuals to continue spreading between. This is like removing enough stepping-stones from a stream crossing to leave the pneumococcus stranded.

Herd protection works for a number of infectious diseases where the bacteria or virus spreads between people, like measles and polio. With herd protection the minority are protected by the majority. The critical fact is that not everyone can be vaccinated for legitimate medical reasons and these people need to be shielded by having the population around them vaccinated.

Babies under a few months old can get pneumococcal infections like meningitis with devastating consequences yet are too young to be vaccinated, so herd protection is their strongest defence. Additionally, since a pneumococcal vaccine has been given routinely to children in the UK, another group of people vulnerable to pneumococcal infections such as pneumonia have benefited: the elderly. By vaccinating enough children we can protect their grandparents and elders too.

Unfortunately, unvaccinated individuals are not distributed evenly in populations. If the pneumococcus gets to a community where vaccination is low, the bacteria can be harboured in the unvaccinated cluster, moving from person to person like a fugitive searching for its next susceptible victim. The key is that although an unvaccinated child might not get sick themselves, they can pass it to others who could succumb to pneumococcal infection.

An international fugitive

The pneumococcus respects no political borders and is found all around the world. Lower income countries often have higher rates of colonisation and infection and can least afford the vaccines. Herd protection plays a key role here as it reduces the number of vaccines they need to purchase; just enough to block its spread.

Pneumococcal vaccination is complicated further by the >90 different types that exist. We can only vaccinate against a few of them: the most infectious and those most resistant to antibiotics. As we stop the types targeted by the vaccine from circulating and causing infections some of the remaining types partially take their place.

The pneumococcus is a moving target. For this reason we need to constantly monitor which types are causing infection or circulating in healthy individuals all around the world. This is why the Global Pneumococcal Sequencing project is sequencing the biggest ever collection of pneumococcal genomes from all over the world, trying to use genetics to understand changes in the pneumococcal population as it responds to the introduction of vaccines.

NB. I refer to herd protection here rather than herd immunity as technically in the case of the pneumococcal vaccine no active immunity is gained by the unvaccinated, instead the surrounding herd protects them.

Rebecca Gladstone is a Senior Bioinformatician in the Pathogen Genomics group at the Wellcome Trust Sanger Institute, where she is currently working on a global collection of 20,000 pneumococcal genomes to assess pneumococcal vaccine impact. Rebecca is interested in learning how to better share the research findings with the public. She Tweets as @becctococcus.

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