Henoch-Schönlein purpura (HSP), now formally called IgA vasculitis, is the most common form of blood vessel inflammation in children. It affects roughly 20 out of every 100,000 children per year, making it uncommon but far from the rarest childhood condition. In adults, it is genuinely rare, striking only about 1 to 2 per 100,000 people annually.
Incidence in Children vs. Adults
HSP is anywhere from 2 to 33 times more common in children than in adults, depending on the population studied. The peak age falls between 4 and 6 years old, where the annual incidence jumps to about 70 per 100,000 children in that narrow age window. By the time someone reaches adulthood, the rate drops to roughly 0.8 to 1.8 per 100,000, which places adult HSP in the same ballpark as many conditions classified as rare diseases.
Boys are diagnosed more often than girls, with a male-to-female ratio of about 1.4 to 1.7 for every 1. Researchers don’t fully understand why boys are more susceptible, but the pattern holds consistently across studies in different countries.
Rates Vary by Geography and Ethnicity
Where you live and your ethnic background both influence how common HSP is. South Korea reports one of the highest rates in the world at nearly 56 per 100,000 children under 18, with the peak age group (4 to 6 years) reaching about 122 per 100,000. European countries report lower figures: around 6 per 100,000 in Croatia and the Netherlands, up to 20 per 100,000 in parts of the UK, and about 18 per 100,000 in southern Sweden.
Ethnicity plays a measurable role as well. White and Asian children develop HSP at 3 to 4 times the rate of Black children. Geographic clustering has also been observed. In Croatia, cases are not evenly distributed but tend to concentrate around certain regions, suggesting that local environmental factors like specific infections may be driving outbreaks.
What Triggers Most Cases
HSP has a strong environmental component, and infections appear to be the most important trigger. About 37% of childhood HSP cases are potentially attributable to a common type of pneumonia-causing bacteria, while roughly 26% are linked to the strep bacteria responsible for strep throat. Another 17% of cases may be triggered by rhinovirus or enterovirus, the bugs behind many common colds. No other pathogens have shown a significant connection.
This infectious link also explains the seasonal pattern. HSP cases peak in spring, accounting for about 29% of diagnoses, followed by winter at 25%. Summer consistently has the fewest cases. The pattern mirrors the respiratory infection season in most temperate climates, reinforcing the idea that these infections set off the abnormal immune response that causes HSP.
How It’s Diagnosed
Doctors diagnose HSP based on a set of clinical criteria rather than a single blood test. The classification system endorsed by European and international pediatric rheumatology organizations requires the hallmark skin finding (palpable purpura, the raised purple-red spots that don’t fade when pressed) plus at least one of four additional features: abdominal pain, joint pain or arthritis, kidney involvement (blood or protein in the urine), or a skin biopsy showing a specific type of immune protein deposit. Every child with HSP develops the characteristic rash, but the combination of additional symptoms varies from person to person.
Kidney Involvement and Serious Complications
The biggest concern with HSP is kidney damage. About 17% of children with HSP develop some degree of kidney involvement, and it disproportionately affects older children. In one study, none of the children under age 6 developed kidney problems, while about 24% of those 6 and older did. The most common sign is microscopic blood and protein in the urine, which often resolves on its own.
A smaller subset develops more serious kidney complications. In the same study, about 29% of children with kidney involvement progressed to a condition marked by heavy protein loss in the urine, and one child developed acute kidney inflammation with high blood pressure. The reassuring finding is that at follow-up (averaging about 19 months), no children had progressed to chronic kidney disease or kidney failure. Most children recover normal kidney function, though some require longer-term monitoring or blood pressure management.
Recurrence After the First Episode
One episode of HSP does not necessarily mean it’s over for good. Recurrence rates vary widely across studies, from as low as 3% to as high as 66%, but a large 16-year nationwide study found that about 16% of children experienced at least one additional episode. That means roughly 84% of children who get HSP never have it again. The recurrence rate translates to about 7 additional episodes per 100 children per year of follow-up, so even among those who do relapse, episodes tend to be spread out over time rather than happening in quick succession.
Putting the Rarity in Perspective
For a parent of a young child, HSP is uncommon but not something most pediatricians would consider exotic. A typical pediatric practice will see cases, especially during cold and flu season. For adults, it is a different story. With fewer than 2 cases per 100,000 adults each year, an adult diagnosed with HSP is dealing with a condition that many general practitioners rarely encounter. The overall prognosis is favorable in both groups, but the rarity in adults often means a longer path to diagnosis simply because it is not the first condition doctors think of when an adult presents with a purplish rash and joint pain.