A study of Google searches for the word “chickenpox” has demonstrated the effectiveness of vaccination against the disease. Where the varicella vaccine has become part of the standard childhood immunization schedule, search rates drop dramatically.

Varicella zoster, better known as chickenpox, was once so common that it wasn't taken seriously. However, it is occasionally very serious and the aftereffects, in the form of shingles, can be debilitating decades later. The introduction of a vaccine against the virus has led to a reduction in infections, and a predictable backlash from the anti-vaccination movement.

Measuring just how successful the vaccine has been is tricky, however. Unlike many of the other diseases vaccinations prevent, chickenpox seldom leads to hospitalization. Most countries do not require doctors to report cases they see.

In search of a different source of data on chickenpox outbreaks, University of Michigan PhD student Kevin Bakker turned to Google searches for the word chickenpox from 36 countries over 11 years.

After the vaccine became widespread in Germany, Australia, and the United States the number of searches declined. Moreover, those searches that did occur were more evenly spaced through the year, rather than coinciding with seasonal outbreaks. In Spain and Italy, where vaccination is restricted to certain regions, similar changes were almost unobservable.

Bakker confirmed the validity of his method by checking Google searches against national-level data in the five countries – Mexico, Thailand, Estonia, Australia and the United States – that require doctors to report cases of chickenpox and keep nationwide records. In Australia and the United States, where the vaccine is widespread, there was a small but significant correlation between the Google data and the official findings, whereas in the three countries where the vaccine is rare the correlation was very strong, bolstering Bakker's confidence in his methods.

"It is really exciting to see human information-seeking behavior – Google searches – being reduced by vaccination implementation," Bakker said in a statement. "It's a very clear signal, and it shows that the vaccine is having a strong effect."

Bakker's findings are published in the Proceedings of the National Academy of Sciences. Google searches have been used to track the spread of infectious diseases before, and even to give health authorities advanced warnings of outbreaks. However, Bakker is the first to use the technique to demonstrate the effectiveness of a vaccine.

Flu tracking turned out to have unexpected problems but Bakker noted the symptoms of chickenpox are easily recognizable, unlike influenza, where it is a challenge to distinguish between people experiencing the disease itself and other conditions with similar symptoms.

Bakker also showed that chickenpox is most common in spring, irrespective of hemisphere, although China and Japan also had winter peaks.

Bakker used his data to create a model with which he was able to predict the size of outbreaks with considerable accuracy for Australia, although it proved less reliable in matching the severity of disease seasons in Thailand.