The effectiveness of the acellular vaccine against pertussis

A study that was published online on 10/06/2019 in a very respectable journal – “Pediatrics” (PMID 31182549). The authors belong to the medical insurance company “Kaiser Permanente” from the United States, serving up to 4 million clients.
By using the massive database of this company, you can gather retrospective data in an enormous scale and answer a variety of questions with just the click of a button.
In this research the prevalence of pertussis in children under the age of 11 was examined in correlation with their immunity status for the disease (the number of vaccine doses and their age).

Introduction: pertussis vaccine regimen in the USA is (acellular pertussis vaccine) given at 2, 4, 6 and 12 months. The fifth recommended dose is given in the USA between 4-6 years
Since the vaccine rates in the USA is high (literature says around 94% vaccination rate), pertussis in children can suggest either that a child is not vaccinated properly (and therefore got infected) or that the child’s immunity level has decreased.

Methods: data was gathered on about 500,000 children. They were divided into 4 categories, based on their age and number of pertussis vaccine doses that they were supposed to receive:
– Unvaccinated children – 1% of the children.
– Partially vaccinated – children who did not receive all vaccine doses required for their age. 3% of the children.
– Properly vaccinated – 70% of the children.
– Properly vaccinated + one more dose of the vaccine – 26% of the children. I don’t know why and where this population came from, I can only think of a few possible scenarios where a child will get an extra vaccine dose for pertussis. Nonetheless, I’m glad to see this group since it makes the results of this study far more interesting.

Results: 738 cases of pertussis were found in the children recruited to this study (0.2% of the children). The analysis demonstrated a few important findings:
– The chance to contract pertussis was 13 times higher in unvaccinated children as appose to properly vaccinated children.
– The chance to contract pertussis was 1.9 times higher in partially vaccinated children as appose to properly vaccinated children.
– Children with an extra dose of vaccine had half the chance of contracting pertussis as appose to properly vaccinated children.
– Most cases of pertussis (80%) happened in children who were properly vaccinated. In this population, the prevalence of the disease increased as time passed from the last vaccine dose.

In summary:

If analyzing the results together, I think that children who are unvaccinated are at a high risk of contracting the disease – a known and not very exciting fact.
The more interesting finding is that most pertussis cases happens in vaccinated children, meaning that pediatricians have to be aware and diagnose pertussis even in fully vaccinated children. This is a known phenomenon, which pediatricians know and deal with on a daily basis.
But why vaccinated children contract pertussis? Since the acellular vaccine is less immunogenic than the previous vaccine, the immunity level decreases over time. The new acellular vaccine has fewer side effects compared with the old vaccine, but it provides less protection against pertussis.
Practically, Pediatrician must have a high level of suspicion for pertussis, even in vaccinated children, especially if more than 3 years have passed since the last dose of the vaccine was administrated. (For example, it is more common to find a 6-year-old child with pertussis than a 3-year-old).

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