Can Non-b Haemophilus influenzae Spread Between People? Eight Likely Cases, 2011–2018

Haemophilus influenzae (Hi) can cause meningitis and other serious invasive infections. Encapsulated strains fall into six serotypes, a through f; strains without a capsule are nontypeable (NTHi).

Before vaccines in the 1980s, type b (Hib) caused 95%–98% of invasive Hi disease and was the leading cause of bacterial meningitis in children. It spread within households and child care, with a risk of secondary disease ranging from 1.2% in children aged 12–23 months to 6% in infants under 1 — so the Advisory Committee on Immunization Practices (ACIP) recommended preventive antibiotics (chemoprophylaxis) for contacts in some circumstances. Vaccination has since cut invasive Hib disease by about 99%; by 2018 it was only 1.3% of invasive Hi disease. But non-b types, especially type a and NTHi, have been rising — type a by 11.1% a year from 2008 to 2017.

ACIP's 2014 recommendations did not call for chemoprophylaxis after non-b disease, because secondary transmission hadn't been documented. A CDC team led by Sara E. Oliver and Amy B. Rubis looked for it.

How they searched

Using Active Bacterial Core surveillance in 10 U.S. jurisdictions, they looked for clusters: cases of the same serotype in the same county within 60 days (2011–2018), or NTHi cases in the same county within 14 days (2015–2018). Site staff reviewed case investigation records for epidemiologic links, and linked cases were checked by whole genome sequencing; isolates fewer than 10 genetic differences apart were considered closely related. Mother–newborn NTHi pairs in the first day of life were excluded as likely infection before or at birth.

What they found

Source: CDC, MMWR.

Eight likely instances of secondary spread of non-b Hi — none of Hib, perhaps reflecting vaccination and existing prophylaxis. Every pair's bacteria were genetically closely related, and every secondary patient had one or more underlying medical conditions.

The five encapsulated pairs:

All patients survived. Most illnesses were bacteremic pneumonia; one primary patient had meningitis, empyema and septic arthritis, another meningitis.

Moving toward prevention

Other evidence points the same way. In Texas in 2019, two infants with type a meningitis at the same child care facility had genetically identical bacteria. In a 2018 type a outbreak in a small rural Alaska community, close contacts carried the bacteria most often, and no further cases followed chemoprophylaxis. Some jurisdictions already act:

• Alaska (2018) urges clinicians to strongly consider chemoprophylaxis for close contacts of invasive type a disease, especially households with children under 4 or immunocompromised members;
• tribal communities in the Southwest with high type a and b disease have similar policies;
• the American Academy of Pediatrics (since 2018) advises considering prophylaxis for type a cases in households with children under 4 or immunocompromised children, and a similar approach in child care, with public health officials.

Because chemoprophylaxis often has to be offered before serotyping results are in, the authors suggest broader recommendations may be warranted for certain populations, weighed against risks such as more antibiotic use.

Limitations

Secondary spread was likely undercounted: links came only from routine records, not all isolates were sequenced, and the 14-day window for NTHi is shorter than the usual 60. Data ran only through 2018.

Sources

Based on Oliver SE, Rubis AB, Soeters HM, et al., "Secondary Cases of Invasive Disease Caused by Encapsulated and Nontypeable Haemophilus influenzae — 10 U.S. Jurisdictions, 2011–2018," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.