A common seasonal respiratory virus that typically infects school-age children is causing an unusually large spike in cases across Europe and the US this summer. The surge in transmission is a significant danger to certain vulnerable populations, including pregnant people, according to a health alert released by the Centers for Disease Control and Prevention on Tuesday.

The virus is parvovirus B19, aka B19V, which spreads via respiratory droplets and typically causes a mild, two-phased illness in children. The first phase is marked by flu-like symptoms that can include fever, headache, malaise, muscle aches, cough, sore throat, and joint pain. In the second phase, a characteristic "slapped cheek" rash develops, which is also called Fifth Disease or Erythema Infectiosum. By age 20, about 50 percent of people have been infected and have antibodies that can help fight reinfection. And by age 40, the figure rises to 70 percent.

But, for those who have compromised immune systems, are pregnant, or have certain conditions, such as sickle-cell anemia, the virus can cause severe complications, such as severe anemia, myocarditis, hepatitis, or encephalitis. In pregnancy, the virus can infect the fetus and cause dangerous complications and miscarriage.

Usually, the virus spreads with little notice, causing severe disease only rarely. But, in recent months, B19V has risen above the din of seasonal viruses. At the end of last year, for instance, health researchers in Israel reported the country's largest and longest B19V outbreak to date, with the rate of cases rising nine-fold over the rate between 2020 and 2022, and 6.6-fold over earlier years. Pregnant people saw a relative increase in risk during the outbreak.

International surge

In June, researchers in Denmark reported a peak in cases earlier this year that was the highest rate of infection in the past 10 years, 3.5-fold higher than the next highest peak from 2017. But Denmark is not alone. Also in June, the European Centre for Disease Prevention and Control reported that 14 European countries have seen unusually large increases in B19V infections.

According to the US CDC, the surge seemed to arrive in North America in June, though the data is limited. Given its typically mild presentation, B19V is not a nationally notifiable infection in the US, meaning cases are not tracked. However, the CDC does passively collect data on the virus.

For instance, commercial laboratories have reported an increase in test positivity, and the proportion of people with IgM antibodies—an indicator of recent infection—increased among all ages from less than 3 percent between 2022 and 2024 to 10 percent this June. Children between the ages of 5 and 9 saw the greatest increase, going from 15 percent between 2022 and 2024 to 40 percent in June 2024. Among plasma donors, the prevalence of pooled samples with a threshold level of B19V DNA went from 1.5 percent in December 2023 to 20 percent in June 2024. The CDC also said it has gotten anecdotal reports of increases in cases of B19V in pregnant patients.

The CDC calls for clinicians to be more aware of the possibility of B19V infections. For pregnant people and others at high risk, the CDC recommends taking precautions, like masking and vigilant hand hygiene. This is particularly important for any high-risk people in work settings that involve close contact with children, such as schools and daycares, the CDC notes.

It's unclear what is driving the B19V outbreak, but researchers have noted that B19V tends to have small surges every three to four years. With the global debut of SARS-CoV-2 at the end of 2019, many seasonal and cyclical patterns of infectious diseases were disrupted, causing unusual dips and spikes in transmission. B19V transmission may have likewise been affected.