When an unvaccinated child is exposed to the Haemophilus influenzae type B (HiB) bacteria, he or she is at risk of serious illness that can become life-threatening. Infection by Hib bacteria can affect the entire body but one of the most deadly complications is meningitis, an inflammation of the meninges, the three-layer membrane that lines the skull and spinal canal to completely encloses the spinal cord and brain.

Once HiB meningitis develops, a child faces a one in 20 chance of death. One in five experience permanent brain damage or become deaf.

Other infections caused by HiB include bacterial pneumonia, an infection of the throat called epiglottitis, and sepsis, a dangerous infection of the blood. Arthritis-like symptoms are produced when bones and joints are infected, and skin infections can occur, too.

The good news is that there is a vaccine that prevents HiB infections when the vaccine is administered in the first year of life and the vaccine schedule is followed until full immunity is achieved. There’s even evidence that the Hib vaccine reduces a child’s risk of developing acute lymphoblastic leukemia, the form of cancer most commonly diagnosed in children but there has been little understanding how the vaccine protects against this form of childhood leukemia.

From Infection to Leukemia

A recent study, however, indicates the vaccine de-activates enzymes that encourage precancerous cells in the blood to morph into cancerous leukemia cells. Dr. Markus Müschen, of the University of California San Francisco’s (UCSF’s) Department of Laboratory Medicine, led the study which involved mice.

Müschen’s research team knew that children who experience repeated HiB infections are more likely to be diagnosed with leukemia than children who don’t suffer HiB infections. Since the Hib vaccine became widely administered, the rate of childhood leukemia has dropped by 20%, according to Müschen.

The researchers replicated the effect of recurring HiB infection in lab mice and used genetic testing to identify the effect of infection on the enzymes that turn on or off the genes that trigger leukemia development.

HiB Vaccine Is Not the Same as the Flu Vaccine

The HiB bacteria is not related to the virus that causes influenza so the vaccine will not protect against the seasonal viral form of the respiratory disease. The seasonal influenza vaccine (flu shot) available from early fall into late spring each year protects against viral pneumonia only, not the bacterial infection caused by HiB. The Hib bacterium is so named because, at one time, it was thought to produce the same disease as the influenza virus.

How HiB is Spread

Like the influenza virus, the Hib bacteria is spread through mucus and mucus droplets that are spread when someone with the infection coughs, sneezes, or touches other people and shared objects without washing hands after a cough or sneeze. Some people carry HiB without producing symptoms of illness but they can spread the bacteria to others, who may become sick.

Who’s at Risk

Young unvaccinated children are at greatest risk, as are the elderly and people of any age who suffer chronic illnesses that impair the immune system. People undergoing immune-suppressing cancer treatments and recipients of organ transplants are also at high risk of infection.


  1. Müschen, Markus, et al. "Mechanisms of clonal evolution in childhood acute lymphoblastic leukemia." Nature Immunology (2015). Web. 8 June 2015.
  2. "Hib (Haemophilus Influenzae Type B)." Vaccines.gov. US Department of Health and Human Services, n.d. Web. 8 June 2015.


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