By Thomas Fekete, MD, FCPP, Section Chief, Infectious Diseases; Professor, Medicine; Associate Professor, Microbiology and Immunology; School of Medicine, Temple University
Varicella, or chickenpox, used to be a common childhood infection. Today’s grandparents and most parents can describe vividly what it was like to have chickenpox or to care for a child with this viral illness. Since the broad use of varicella vaccine, the number of cases in the United States has dropped dramatically. Even infants under the age of 6 months who are too young to get the vaccine have had a 90% drop in their rate of chickenpox since the introduction and widespread use of varicella vaccine. But chickenpox has not completely disappeared.
Philadelphia, Pennsylvania, is a city that requires evidence of varicella immunization to attend school (with certain exceptions) and maintains a vigorous surveillance of students attending school to see who has developed chickenpox. This tracking allows public health authorities to see the pattern of spread of chickenpox. A study published in the May 1 edition of the Journal of Infectious Diseases examined varicella cases over a 7-year period, during which about 2300 cases of varicella were seen in Philadelphia public school students. Most of them were “sporadic,” meaning that there was no direct association identified between the student and his/her source of chickenpox. However about 1/3 of cases were associated with a known contact at the school in a time frame that is compatible with transmission. Prior vaccination was present in some of the students who developed chickenpox, and while it was not protective enough to prevent the rash from developing, the cases were mild with fewer spots and a shorter duration.
Even the students who received 2 doses of chickenpox vaccine were not completely protected. However there was a dramatic reduction in the rate of chickenpox about half-way through the study period when the schools started to require a second dose of varicella vaccine. Without good surveillance, it might have been hard to figure out that there was a broad transmission of chickenpox or that the second dose reduces this risk tremendously.
A further interesting observation was that people with herpes zoster or shingles were the source of the virus in about 10% of the cases. Herpes zoster is caused by the same virus as chickenpox, but unlike chickenpox, zoster is characterized by local pain, itching, and rash in the area of a nerve where the virus has reactivated and tends to affect only the skin along the distribution of that nerve. As a result, zoster rashes are usually seen only on one side, (e.g., the left side or right side) of the body or face. People with zoster are often thought not to be contagious or to have very limited ability to spread the virus to others. But in this Philadelphia setting, there was transmission of varicella virus to healthy school-age children from otherwise healthy kids or adults with zoster. This reminds us that zoster (shingles) does have the capacity to be transmitted, although the exact mechanism is not known. It is possible that the virus could survive long enough on inanimate objects to be the source of infection to others. The Philadelphia investigators did find traces of the virus on various surfaces. There might also be some varicella virus shedding via respiratory secretions in people with zoster. In some patients with zoster, there are pulmonary features such as symptoms like cough or sputum production and documented by imaging studies such as X-rays and CAT scans. But respiratory spread can occur even without these features since small amounts of virus can be present in the respiratory tract without any clinical or radiographic abnormalities.
Take home lessons:
1) Even the excellent protection afforded by the varicella vaccine may not be enough if exposures persist.
2) The shingles rash may be more than just an annoyance for the sufferer but also a potential source of infection for others.
3) Most cases of chickenpox are now so mild that you don’t have to worry about getting too sick.
4) That second dose of varicella vaccine does seem to prevent many cases of chickenpox.
Bloch KC, Johnson JG. Varicella zoster virus transmission in the vaccine era: unmasking the role of herpes zoster. Journal of Infectious Diseases. 205;9:1331-1333.
Viner K et al. Transmission of varicella zoster virus from individuals with herpes zoster or varicella in school and day care settings. Journal of Infectious Diseases. 205;9:1336-1341. http://jid.oxfordjournals.org/content/205/9/1336.abstract