One of the coolest things about vaccines is that they work on two levels: at the individual level and at the population level. That means that even a vaccine that has a lower effectiveness, say 80%, can still protect more than 80% of the population when enough individuals have received it. This occurs, of course, through herd immunity, illustrated in a helpful animation here.
I’ve heard some claim – such as the uninformed anti-vaccine mom blogger lampooned on a recent Daily Show episode – that herd immunity is a myth. Yet myths don’t typically have evidence clearly showing their reality, and a study in today’s Pediatrics reveals yet again how herd immunity can protect even though who have not been vaccinated against a particular disease.
The study compared rotavirus infections before and after the rotavirus vaccine’s introduction in 2006, and the data after its introduction included hospitalizations for both vaccinated and unvaccinated children. Spoiler alert: rates for BOTH populations went down by quite a bit. In fact, for vaccinated kids, hospitalization rates dropped a whopping 92% or 96% (depending on vaccine). But even among kids who didn’t get the vaccine, hospitalizations for rotavirus in the years after the vaccine was available were 25% to 77% lower (depending on the year) than in the years before the vaccine.
To do the study, CDC researchers used insurance claims data from the 2001-2011 Truven Health MarketScan Commercial Claims and Encounters Database, which includes insurance claim information from a variety of public and private health plans (excluding Medicaid). They gathered several sets of data:
- vaccination coverage of RV5 (the five-strain rotavirus vaccine) between January 2006-June 2011
- vaccination coverage of RV1 (a one-strain rotavirus vaccine) between January 2006-June 2011
- patients under 5 years old seen in the ER or in inpatient or outpatient settings for diarrhea-related conditions
- total hospitalizations related to rotavirus during each year from 2007 through 2011
- total hospitalizations related to rotavirus during the five years (2001-2006) before the rotavirus vaccine was introduced
- hospitalizations of children under 2 years old for rotavirus, who did not receive the rotavirus vaccine
More than 406,000 children under age 5 had records in the insurance claims data, and 58% of them had received at least one dose of RV5 by the end of 2010. Another 5% had received at least one dose of RV1 by then.
During the entire time frame studied, nearly 54,000 hospitalizations, nearly 243,000 ER visits and approximately 1.8 million outpatient visits were related to a diarrhea condition in a child under 5 years old. The only pattern seen from 2001-2006, before the rotavirus vaccine was introduced, was that these visits peaked in February and March – also the high season for rotavirus.
Yet from 2007-2011 (after the vaccine had been introduced), these Feb-March peaks were blunted. For kids under 5 – and especially kids aged 1 – visits to the ER, outpatient visits and hospitalizations were lower for each rotavirus season between 2007 and 2011 except for 2008-2009 ER and outpatient visits, which makes sense since there was a nationwide surge in rotavirus infections in 2008-2009.
Meanwhile, the researchers found that “Overall, reductions in rotavirus-coded hospitalization rates correlated with vaccine coverage across age groups, but the magnitude of reductions tended to exceed the vaccine coverage in each age group and year.” In other words, the higher vaccine coverage was, the bigger a drop in hospitalizations was seen – but the hospitalization decrease was proportionally larger than the immunization coverage for the rotavirus vaccine. That means unvaccinated kids’ hospitalization rates had to be dropping as well.
And that’s exactly what they found when they analyzed the data.
It’s not terribly surprising that hospitalizations were 92% lower for kids who got at least one dose of RV5 and 96% lower for kids who got at least one dose of RV1. After all, the vaccine should have been protecting most of those kids from the virus.
We know it was the vaccine protecting these children because the researchers compared the rates between vaccinated and unvaccinated children. For example, kids under age 1 who got the vaccine had 87% lower hospitalization rates for rotavirus than unvaccinated kids under age 1. Among 4-year-olds, the vaxed kids had 81% fewer hospitalizations than the unvaxed kids.
But the part that’s remarkable – and that shows how effectively herd immunity works – is that rates of rotavirus hospitalizations for UNvaccinated children in 2007-2008 (the year of the peak) were half of what they were during the five years before the vaccine was introduced. The 2009-2010 rotavirus hospitalization rates for unvaccinated kids was 77% lower than the 5-year average before the vaccine. For 2010-2011, the rate was 25% lower.
Put simply, fewer kids needed to be hospitalized for rotavirus EVEN IF they didn’t get the vaccine. How did that happen? Well, it wasn’t sanitation, folks. And no other major changes occurred between the five years before the vaccine and the several years after the vaccine except… the vaccine.
When enough kids were getting the vaccine, which was obviously working since hospitalizations among vaccinated kids decreased so sharply, it prevented the virus from spreading throughout the population. Vaccinated kids who weren’t getting rotavirus because they were protected ALSO weren’t passing it along to other kids, vaccinated and unvaccinated.
The kids who were unvaccinated benefited from those who got the vaccine. Meanwhile, however, those who were unvaccinated? They were the ones more likely to contract rotavirus… and therefore to pass it along to other kids.
Put simply: vaccinated kids were protecting themselves AND others. Unvaccinated kids were at risk AND putting other kids at risk.
The researchers reported a variety of other stats and findings (including cost savings from the reduced illnesses), but I don’t want to overwhelm readers with a long list of numbers. The biggest concern with rotavirus is hospitalization, and it’s clear that hospitalizations dropped significantly precisely when the vaccine arrived on the scene, both for the kids who got the vaccine and those who didn’t.
The study was conducted by researchers at the CDC with no outside funding, and the seven authors had no disclosures, which means they have no financial ties (past research funding, advisory boards, travel funds, consulting, etc.) with pharmaceutical companies. While sponsorship of a study by a pharmaceutical company does not invalidate its findings by any stretch, it is important that we have studies both from industry and from government health entities, such as the CDC to ensure a consensus about findings regarding specific vaccines.
