Dr Muiris Houston: Why infectious diseases are rising worldwide

Waning immunity is now a scientific fact for whooping cough, measles and mumps

Among children who get all doses of DTaP vaccine on schedule, effectiveness is very high within the year following the final dose – at least nine out of 10 children are fully protected

Among children who get all doses of DTaP vaccine on schedule, effectiveness is very high within the year following the final dose – at least nine out of 10 children are fully protected

 

Despite having effective vaccines, infectious diseases such as mumps, measles and whooping cough are on the rise globally. And while false news about vaccine safety – leading to vaccine hesitancy – has undoubtedly played a part, are there other reasons for this unwelcome trend?

The latest figures from the World Health Organisation suggest there has been a 300 per cent increase in the number of measles cases worldwide in the first three months of 2019. It noted that many countries are in the midst of sizeable measles outbreaks, with all regions of the world experiencing sustained rises in cases. Coming on top of sequential increases in measles cases in the previous two years, it is clear we have a problem with a potentially fatal infectious disease, that – until relatively recently – we thought could be eradicated.

Could waning vaccine immunity be one of the reasons?

After decades of declining pertussis (whooping cough) and mumps incidence, for example, recent years have seen a resurgence of each disease despite maintenance of high vaccine coverage. Cases have mainly been among young-adult age groups. For mumps, protection appears to wane over decades, prompting the use of additional doses of vaccine for outbreak control. And doctors are seeing an increasing number of adults with the interminable “whoop” of whooping cough.

Falling immunity

Earlier this month, Toronto researcher Dr Jeff Kwong and his colleagues published a study in the journal Vaccine suggesting that the vaccine for pertussis works well at protecting people from the disease during the first decade of life, but that immunity later falls.

It seems vaccination is more complex than we might have thought

Pertussis vaccines are effective, but not perfect. They typically offer good levels of protection within the first two years after getting the vaccine, but then protection decreases over time. Public health experts call this “waning immunity”. Similarly, natural infection may also only protect you for a few years.

When it comes to waning immunity, it seems that the acellular pertussis vaccines (DTaP and Tdap) used now may not protect for as long as the whole-cell vaccine (DTP) doctors used to use. Whole-cell pertussis vaccines are associated with higher rates of minor and temporary side effects such as fever and pain and swelling at the injection site. Serious neurologic adverse reactions, including chronic neurological problems, occurred rarely among children who had recently received whole-cell vaccines, prompting the development of the acellular version.

Full protected

In general, DTaP vaccines are 80 to 90 per cent effective. Among children who get all doses of DTaP on schedule, effectiveness is very high within the year following the final dose – at least nine out of 10 kids are fully protected. There is a modest decrease in effectiveness in each following year. In the first year after getting vaccinated with Tdap, it protects about seven out of 10 people who receive it. About three or four out of 10 people are fully protected four years after getting this version of the whooping cough vaccine. And receiving the newer (acellular) version of the vaccine was associated with twice the odds of contracting whooping cough compared with those who received the older vaccine when they were infants.

Resurgence in mumps

Meanwhile, Scottish researchers have found evidence that the resurgence in mumps cases is due to waning immunity. They wanted to know why a vaccine-preventable disease is re-emerging in highly vaccinated populations. In 2015 some 67 per cent of those infected with mumps in Scotland were fully vaccinated individuals. Most of these cases occurred in adolescents and young adults, in contrast to the pre-vaccine era where outbreaks were among primary school children.

So it seems vaccination is more complex than we might have thought. What was traditionally viewed as a “done and dusted” task for childhood may now need to extend to adult life. With waning immunity a scientific fact for whooping cough, measles and mumps, booster vaccinations throughout life are likely to become part of future national immunisation programmes in Ireland.

mhouston@irishtimes.com

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