As San Diego, Detroit, Louisville, and Salt Lake City continue to contend with the aftermath of hepatitis A virus (HAV) outbreaks, public health experts are wondering why data from the Centers for Disease Control and Prevention (CDC) appear to minimize the problem, The Huffington Post reports.

According to a recent HuffPo analysis of CDC and California Department of Health data, from 2016 to 2017, hepatitis A cases have skyrocketed 48.7 percent nationwide. Meanwhile, the CDC has reported only a 5.4 percent increase in its weekly Morbidity and Mortality Weekly Report (MMWR) — which has led to some head-scratching among advocates.

So far, the CDC has declined to comment on the 48.7 percent figure. But public health experts say a number of factors are driving the outbreak: an increase in the number of homeless people (who are at greater risk for the infection because of lack of access to clean water and sanitary facilities), a rise in injection drug use and a lack of access to health care among many vulnerable populations.

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Meanwhile, as states and cities across the country are battling these outbreaks, advocates have been frustrated by their inability to draw attention to them.

Historically, hepatitis A infections in the United States have been transmitted via contaminated food products, usually imported from other countries. Small outbreaks have also occurred among men who have sex with men in Europe and America. But this is one of the first times that lack of access to sanitation services — a shocking concern for a developed country — has led to an HAV outbreak.

What’s more, health experts say many Americans don’t know they might be susceptible to hepatitis A infection. Although a preventive vaccine is available, it was only approved for use in 1995, meaning that adults older than age 30 have likely missed out on the shot. 

As for the missing CDC data, the federal agency says a mishap in the transfer of information from the California Department of Health’s database might have led to the mistake and reminded reporters that MMWR case counts are not generally considered the final counts for outbreaks.

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