The Bexsero© vaccine

Bexsero© is the new Neisseria meningitidis serogroup B (MenB) vaccine being introduced as part of the childhood vaccination schedule.

Meningitis and Bexsero©

Meningitis is an extremely dangerous disease, caused by infection of the otherwise sterile membranes around the brain and spinal cord. Symptoms of meningitis begin with fever, nausea and headache but can quickly progress to coma and death. A major cause of meningitis is the bacterium Neisseria meningitidis (the 'meningococcus'). N. meningitidis can be placed into different serogroups based on the structure of its capsule (a slimy layer surrounding many bacteria). Serogroup B (MenB) bacteria are particularly dangerous to young babies (up to one year), and are the highest cause of mortality in this age group. Bexsero© is the new MenB vaccine developed by Novartis. The vaccine was introduced for use in babies as part of the childhood vaccination schedule in September, 2015.

How does the Bexsero© vaccine work?

Vaccines work by injecting components (known as antigens) of dangerous bacteria  into the body. Antigens on their own cannot cause disease. Instead, they prepare the body to fight infection by helping the immune system recognise disease-causing cells. You can learn about the basics of vaccines over at the 'Vaccines' page!

If the body has seen the antigen before, it can deal with infection far more quickly, killing the bacteria before it has a chance to do any damage.

Why has it been so difficult to develop a MenB vaccine?

Typically, meningococcal vaccines have used as an antigen, the bacteria’s capsule, a sugary layer surrounding the cell (the MenACWY vaccine for example). However it has not been as easy to develop a vaccine for MenB as its capsule resembles a molecule on the surface of developing human cells. If we took this approach with a MenB vaccine, our immune system would recognise the bacterium as one of our own cells and wouldn’t protect us. In the worse case scenario, this approach may even turn the immune system against our own cells (autoimmunity).

MenB capsule

The diagram above highlights why the capsule of MenB is not a suitable antigen for use in vaccination. It's similarity to developing human cells means our immune system recognises the antigen as self.

For MenB then, researchers at Novartis tested a variety different MenB antigens for their immune response. Four surface antigens were picked as the most appropriate candidates for use in the vaccine. These are:

  • PorA
  • NadA
  • fHbp
  • NHBP

You can find more information on these antigens on the Bexsero© website.

To understand the effectiveness of Bexsero©, we first need to understand a bit about the variation between different strains of MenB in terms of their antigens.

  • (1) Each of the Bexsero© antigens can have slight differences in their structure. Antigens of these different structures are grouped into ‘families’. Each of the Bexsero© antigens are from one family, and for the vaccine to protect us, at least one of the antigens in the vaccine, must match the antigen on the infecting MenB cell. This way the immune system can easily recognise the harmful bacteria based on the antigen it has already seen from the vaccine.

fHbp provides an additional layer of complexity:

  • (2) In addition to the different families, there are also differences in the amount of fHbp we can see on the MenB surface between strains. In some cases, even if the fHbp antigen is the right family, there may not be enough of it on the surface for the immune system to recognise. This can be problematic, as sometimes fHbp is the only protective antigen which matches the vaccine.

Do we need to worry about the vaccine?

MenB, like all bacteria is a constantly evolving and making a vaccine that never fails is very difficult. The antigen families given in the Bexsero© vaccine are predicted to cover more strains of the disease than any other. Scientific tests (MATs and SBAs) have estimated that the vaccine will protect us against between 73-88% of infecting MenB strains. Despite this, there is still a very small chance that we may be infected by MenB strains capable of escaping the vaccine and causing disease.

In what situation might the vaccine not work?

Following vaccination, it takes short period of time for the body to recognise the antigens and develop ‘immune memory’. Therefore, if we are infected with MenB in the first few weeks after vaccination it is still possible that we may develop disease. Following the first few weeks, for us to be protected, at least one of the antigens on the bacterium must be a match for at least one of the antigen families given in the vaccine. Finally, if fHbp is the only antigen that matches, there must be enough of it on the cell for the immune system to recognise.

This does not mean that we should be worried about the Bexsero© vaccine. The combination of vaccine coverage and protocols in place for suspected infection greatly minimise the chance of MenB disease progressing. This does however mean that despite vaccination, we should remain vigilant of the symptoms of meningitis and contact our doctors if we experience them.

Bexsero failure 2

Read about how we are developing tools to assess the effectiveness of Bexsero© here.

 

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