There was a time when chickenpox was such a common childhood affliction, the sudden appearance of a few red spots-turned-blisters was almost instantly recognizable to parents and doctors.

With more than 4 million cases each year in the early 1990s in the United States alone, nearly all American children experienced its rash at some point. What was a mild inconvience for many was devastating for some, with thousands hospitalized and hundreds dying from the highly infectious disease.

Chickenpox remains common in a number of populations around the world, even in wealthy countries like Sweden where vaccination against the virus is free but voluntary. Yet thanks to a widely successful vaccination campaign starting in 1995, the disease is now so rare in the US that doctors are misdiagnosing it half the time, according to a report from the Minnesota Department of Health.

The current scarcity of chickenpox cases in the US is undoubtedly a good thing. The country's nationwide childhood vaccination effort reduced case numbers, hospitalizations and deaths in under 20-year-olds by more than 97 percent.

Having so few severe case studies to study could be presenting medical specialists with a diagnostic challenge, however.

Some people who have been vaccinated against chickenpox can still get the disease, usually with milder fevers and fewer or no blisters, just red spots. This occurs more often in people who've only had one dose of the vaccine, not the recommended two.

Since the milder-looking 'breakthrough' chickenpox looks different to what doctors might be expecting of the once-rampant childhood disease, it can be difficult to diagnose just by looking at the red rash or spots.

But accurate diagnoses of chickenpox are needed to shape public health policies, respond to outbreaks, evaluate vaccination programs, and understand the disease epidemiology, which has shifted dramatically since 1995.

In 2016 the Minnesota Department of Health (MDH) expanded its laboratory testing for chickenpox and monitored the outcomes. The department provided free testing at its public health labs for cases of suspected chickenpox, including cases identified in medical clinics, and those reported at schools or childcare. Free at-home testing kits were also available to families.

The investigation found that of 420 patients with a suspected chickenpox infection who provided specimens to MDH's labs between December 2016 and March 2023, only 37 percent tested positive for varicella-zoster – the virus that causes chickenpox.

One-fifth of those positive cases were examples of 'breakthrough' chickenpox, having received at least one dose of the chickenpox vaccine, and around half of the patients tested by MDH were fully or partially vaccinated.

More specifically, among 208 patients whose doctors suspected they had chickenpox after examining them at a medical facility, only 45 percent (or 95 people) tested positive for the disease. Another 26 people tested positive for an enterovirus, and a sole person was found to have HSV-1, a different type of herpes virus related to chickenpox.

That leaves 86 Minnesotans who returned a negative result counter to their doctor's suspicions, and might've had some other skin lesions or infection that was mistaken for chickenpox.

"These findings suggest that the clinical diagnosis of varicella can be unreliable, especially in vaccinated patients, and underscore the importance of laboratory confirmation of varicella," MDH's Alison Ruprecht and colleagues conclude in their report.

It's important to note, however, that test results from MDH's labs represented only a fraction of lab-confirmed cases of chickenpox in Minnesota during this 6-year period, so their data doesn't capture the full picture of doctors' diagnoses.

While the findings ought to prompt doctors to review their testing practices, they are a by-product of a successful vaccination campaign that slashed chickenpox case numbers in the US.

Perhaps one day, the only cases doctors will ever get to see will be those preserved in historic records.

The public health surveillance effort has not been peer-reviewed like a research study, but can be accessed here in the CDC's Morbidity and Mortality Weekly Report.