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Investigation and Outcomes

To determine whether increased B19 activity continued from 2024 into 2025, ÐÇ¿ÕÓéÀÖ¹ÙÍø analyzed data on serum B19-specific immunoglobulin M (IgM) antibodies, a marker of recent infection. Data were obtained from ÐÇ¿ÕÓéÀÖ¹ÙÍø’s National Syndromic Surveillance Program (NSSP)* and originated from tests conducted by a large commercial laboratory. IgM antibodies were assayed using an enzyme immunoassay approved by the Food and Drug Administration; an index value >1.1 indicates antibody detection. Laboratory test data were consistently received from all 10 U.S. Department of Health and Human Services’ regions^† during the study period. Region 2 (New Jersey, New York, Puerto Rico, and the U.S. Virgin Islands) was overrepresented compared with the U.S. population.

The weekly number and proportion of positive IgM test results among children and adults submitted to NSSP during January 1, 2023–May 10, 2025, were summarized by week. Positivity ratios (PR) were calculated by dividing the proportion of positive IgM test results in the first two quarters of 2025 by the proportion of positive IgM test results in the same quarters of 2024. PRs with 95% CIs were examined overall and by age group; 95% CIs that excluded the value of 1.0 were considered statistically significant. This activity was reviewed by ÐÇ¿ÕÓéÀÖ¹ÙÍø, deemed not research, and was conducted consistent with applicable federal law and ÐÇ¿ÕÓéÀÖ¹ÙÍø policy.^§

In 2024, the proportion of positive IgM test results increased from 3.3% during mid-February (week 7) to a peak of 9.6% in late June (week 27) and then decreased to a low of 2.0% during late October (week 44) (Figure). The proportion of positive IgM test results increased from 2.8% in mid-November 2024 (week 46) to 7.3% in early May 2025 (week 19). The number of weekly IgM tests performed in 2025 (mean = 1,401; 95% CI = 1,333–1,469) was similar to 2024 (mean = 1,328; 95% CI = 1,278–1,377). The proportion of tests ordered for females and males was almost equal for all age groups except 15–44 years (93% female) and ≥45 years (66% female).

Compared with 2024, the proportion of positive IgM test results in 2025 was significantly higher in both the first (PR = 1.8; 95% CI = 1.6–2.0) and second quarter (PR = 1.2; 95% CI = 1.1–1.4) (Figure). Except for children aged <5 years, PRs of all age groups were significantly higher in the first quarter of 2025 than in 2024, with the highest estimate in children and adolescents aged 10–14 years (PR = 2.2; 95% CI = 1.5–3.0). In the second quarter, PRs among those aged 10–14 years and 15–44 years were significantly higher in 2025 than in 2024.

Preliminary Conclusions and Actions

B19 IgM test data for persons receiving testing through May 10, 2025, indicate increased, sustained B19 transmission, particularly among persons aged 10–14 and 15–44 years, which includes women of reproductive age. These estimates might be an undercount because they relied on clinician testing; parvovirus B19 infection, especially mild infection, is likely far more prevalent than represented in these data. Early identification of B19 infection can prompt early detection and treatment of severe anemia, helping to reduce adverse fetal outcomes and severe disease in persons who are immunocompromised or have chronic hemolytic blood disorders. For this reason, health care providers should consider testing 1) pregnant women who might have been exposed to B19 and 2) persons at increased risk for severe disease and who have signs and symptoms including fever, rash, arthropathy, or unexplained anemia with low reticulocyte count (3). Health care providers caring for pregnant patients should remain vigilant for signs of reduced fetal movement or evidence of hydrops, which could be associated with B19 infection (4,5). Pregnant women and persons at increased risk for complications from B19 infection might consider using additional prevention strategies, such as wearing a mask around other persons.