Health and Science News for Parents

When lucky 13 isn’t enough to protect some kids from disease

written by Tara Haelle

The tricky thing about pathogens is that some of them can wear a lot of different costumes. The sexually transmitted virus human papillomavirus (HPV), for example, has more than 100 strains. Fortunately, not all of these cause cancer, so the two HPV vaccines available target the strains that cause the most cancer.

But sometimes when vaccines target some strains of a bacteria or virus, the strains that weren’t included in the vaccine become more common. That phenomenon is partly the reason the PCV13 vaccine was developed to replace the PCV7 vaccine (which, by the way, prevented about 65,000 deaths a year in the US after it arrived). Both protect against the Streptococcus pneumoniae bacteria, which causes pneumococcal disease, but PCV13 protects against six additional strains. The PCV7 had covered the strains that initially caused more than 80% of illness, but after it was introduced, the illness caused by strains not covered increased by 140%. So, enter PCV13. Now a new study in Pediatrics calls into question whether protection against 13 strains is enough either.

The PCV13 vaccine is successfully preventing pneumococcal disease in more children. But it may not be enough for the kids already at risk with other health problems. Photo by Julia Freeman-Woolpert

The PCV13 vaccine is successfully preventing pneumococcal disease in more children. But it may not be enough for the kids already at risk with other health problems. Photo by Julia Freeman-Woolpert

Although pneumonia is the most common illness to result from this disease, those who become sick from this bacteria can also develop meningitis or blood stream infections, all of which can be fatal. That’s why it’s included on the CDC’s recommended schedule for childhood immunizations. And that’s why it’s important to understand whether we’re successfully protecting against enough of the strains.

There is another pneumococcal vaccine, PPSV23, which protects against almost twice as many types of pneumococcal bacteria. It’s recommended for adults aged 65 and older and for anyone who is at a higher risk for disease, including those who are smokers or have asthma. This vaccine is not currently recommended for everyone, but the this new study shows why it may be helpful for those with underlying conditions who are at a higher risk for getting sick.

To learn the effect the PCV13 vaccine was having on pneumococcal disease, researchers looked at all the cases of the disease among children aged 5 and younger in Massachusetts a couple of years before and after the vaccine was introduced.

The good news is that the total number of pneumococcal disease cases did drop: there were 168 cases in 2007-2009, before PCV13 came on the scene, and there were 85 cases after PCV13 was introduced. That means the vaccine sliced in half the number of kids getting sick – not a bad result for bumping up from the old PCV7 vaccine. And the reduced illness did appear to result from the new vaccine because the strains covered by PCV13 dropped 18 percent in the two years after it was used.

However, proportionally, more of the kids who got sick after the vaccine came out ended up hospitalized, compared to the percentage of kids who got sick before the PCV13 vaccine. That is, 58% of those 85 kids post-PCV13 ended up in the hospital with pneumococcal disease, compared to 51 percent of the kids who got sick when it was just PCV7 being used.

Be careful, however, because the difference in percentages can be tricky. Yes, a higher percentage of kids *who got sick* were hospitalized post-PCV13 than pre-PCV13, but the OVERALL percentage of kids who were hospitalized with pneumococcal disease was still much lower after the newer vaccine.

Consider this: 58% of 85 kids is 50 children who were hospitalized AFTER the PCV13 vaccine came out. Before the vaccine, 51% were hospitalized… but that’s out of 168 kids, which is 85 children. It’s important to understand the difference or else you’ll hear folks claiming that more kids got sick after the newer vaccine came out, which is not at all the case. The total amount of kids hospitalized with pneumococcal disease dropped by 41% after PCV13 was released (that’s 85 – 50, which is 35, divided by 85, which is a 41% decrease). It’s just that AMONG the kids who got sick before and after, a bigger percentage of the “after” kids required hospitalized than the percentage of “before” kids.

Consider the first statements the researchers make under the Conclusions: “We present population-based data that show a statistically significant decrease in rates of invasive pneumococcal disease and of invasive pneumococcal disease caused by vaccine-related serotypes since the introduction of PCV13. The data support a reduction in overall morbidity for invasive pneumococcal disease, with a trend toward children with underlying illness, and show that those with co-morbid conditions have greater morbidity.”

In plain English: bringing PCV13 onto the scene led to less illness, including less illness among the kids with underlying conditions. Yet, those kids with other conditions are still getting the sickest and needing hospitalization the most when they do end up developing pneumococcal disease.

So what about that last part? Well, the researchers found that the kids hardest hit by the disease were those with underlying conditions, especially after PCV13 was released. Before PCV13 came out, 20% of the kids who got sick had other underlying conditions. After PCV13 came out, 24% of the kids who got sick had other conditions. This doesn’t necessarily mean more kids with underlying conditions are getting sick, but it does mean the burden of disease is shifting because more healthy kids are escaping illness.

As would be expected then, among the kids who got sick from strains not covered by PCV13, 28 percent had underlying conditions and 17 percent did not. So, the burden of overall disease and the burden of disease caused by non-vaccine strains is shifting to the kids already dealing with other health issues. Accordingly, four out of every five kids with underlying conditions who got sick from a non-vaccine strain were hospitalized, compared to half of the kids without underlying conditions.

Whew! That’s a lot of numbers. What does it all mean?

First, the pneumococcal vaccine PCV13 is working: half as many kids got sick with pneumococcal disease after it was introduced, and almost half as many ended up in the hospital.

Second, the strains not covered by the vaccine are hitting the kids with other health problems the hardest. So, healthier children appear to be able to fight off these other strains better.

Third, when these strains do succeed in sickening kids, it’s not pretty: more of them end up in the hospital, especially if they had an underlying condition. And the less healthy kids stay in the hospital longer too: those with underlying conditions stayed a median of 3 days while those without such conditions stayed only a median half day. (Remember median is the middle number, not the average, so half of the kids with underlying conditions stayed less than 3 days, and half stayed more than 3 days.)

But here’s the kicker: the differences in percentages of kids hospitalized before and after the vaccine AND the differences in percentages of sick kids with and without underlying conditions… were not statistically significant. That means we can’t rule out chance as the reason for the different percentages. This actually makes sense since we’re talking about so few kids here — a total of 253 children isn’t enough to get really strong statistical results. The bigger the number, the more certain we can be about the math. It’s great that not a lot of kids were getting sick, but it also makes it harder for us to learn whether the introduction of PCV13 is really influencing the other strains’ prevalence.

Given how close to each other the before and after percentages of hospitalization are, and the fact that more of the kids getting sick are ones with underlying conditions, it might be that the higher hospitalization rates are simply a result of the fact that more of the sick kids are those with other health problems now. It makes sense that these kids are likely to be hit harder by the disease and therefore end up in the hospital. So if more of these kids with other issues are getting sick, we would EXPECT to see higher hospitalization rates. And we do.

And what’s the solution? Well, we already have a vaccine that covers more than just the 13 strains in PCV13. The authors suggest that PCV13 alone may not be enough to protect the kids who have other health issues going on, so maybe recommending they get the PPSV23 at an early age will help reduce the disease burden they’re taking on.

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One Response to “When lucky 13 isn’t enough to protect some kids from disease”

  1. Great post. Thanks for identifying the importance of this vaccine and for clarifying what the numbers actually mean. It is so important to fully understand what the statistics are telling us, otherwise they can (and often are) misinterpreted and skewed to scare parents away from vaccination.

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