Human Metapneumovirus Season Identified

Beth Skwarecki

April 04, 2016

Human metapneumovirus (HMPV) begins circulating in the winter and lasts until spring, judging from laboratory tests submitted to a national surveillance system from 2008 to 2014. The HMPV season overlaps with the respiratory syncytial virus (RSV) and influenza seasons, but peaks after the other two viruses, researchers report in an article published online April 4 in Pediatrics.

"Currently there is no vaccine for HMPV. Thus, describing HMPV circulation in the United States in the context of RSV and influenza may help clinicians to prioritize diagnostic testing, identify an etiologic agent, manage patients clinically, and choose appropriate prevention strategies," write Amber K. Haynes, MPH, and colleagues from the Centers for Disease Control and Prevention, Atlanta, Georgia.

HMPV was first identified in 2001 and is associated with an estimated 20,000 hospitalizations each year in the United States among children aged 5 years and younger. Elderly and immunocompromised individuals are also at increased risk for the disease, which causes both upper and lower respiratory tract infections, including bronchiolitis and pneumonia.

Because the symptoms are clinically similar to those of infections caused by RSV or influenza, the authors urge patients be tested to distinguish between the infectious agents. Differences between the viruses lie in the populations most susceptible to severe infections and the management of those infections.

Haynes and colleagues examined the seasonality of the virus through reports to the National Respiratory and Enteric Viruses Surveillance System, which also tracks RSV and influenza. Seasons are measured from July to the following June. By the 2008 to 2009 season, enough samples of HMPV were reported (more than 70,000) that it became possible to analyze patterns for this virus.

Among laboratories that reported HMPV at least 36 weeks out of the year, 945,836 tests were reported from July 2008 to June 2014. Positive results occurred in 3.6% of tests (3.9% of polymerase chain reaction tests and 3.1% by antigen detection). Among laboratories that consistently reported the other viruses, 15.3% of the 1,839,877 tests for RSV were positive, as were 18.2% of the 2,206,654 tests for influenza. (The study period included the 2009 H1N1 pandemic, but the authors removed the 2008 - 2009 and 2009 - 2010 seasons when calculating the seasonality of influenza.) Children's hospitals made up 28.9% of the laboratories consistently reporting HMPV tests.

"The weekly proportion of specimens positive by antigen detection or [polymerase chain reaction] methods for HMPV show definitive seasonal patterns each year," the authors write. Positive results occurred year-round, with a low of less than 1% from July to November and a high of 6% to 16% in March and April.

During the 6-year period, the median onset of HMPV season fell in the first week of January, varying from late November to late February in different years. Onset was defined as the first of 2 consecutive weeks when the proportion of positive weekly tests exceeds 3%. Seasons for RSV and influenza use a similar definition with a 10% positivity threshold.

The median end of the season was in mid-May, with individual seasons ending from late April to early July. The median peak was in late March.

All three viruses circulated concurrently, but the seasons for each were different: RSV was the most consistent, arriving before influenza, and HMPV typically arrived later. In each year studied, the HMPV start, peak, and end occurred after the corresponding points of the RSV season. Influenza's season was more variable.

All three viruses averaged 21 to 22 weeks for their season. The median time between the onset of RSV and that of HMPV was 8 weeks; the peaks were separated by 9 weeks.

A major limitation of the study, the authors note, is that it depended on a virus surveillance system that is passive and voluntary. The participating laboratories may also vary from season to season. RSV's status as a newer concern than the other viruses may mean it is under-represented, and health professionals likely do not consider or test for HMPV as often as for the other viruses. Although the reporting laboratories came from many areas of the United States, there were not enough data to analyze regional variation.

The authors have disclosed no relevant financial relationships.

Pediatrics. Published online April 4, 2016. Abstract


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