Risk of infection
Varicella is highly infectious and infection is both airborne and through contact (10). Localised herpes zoster is considered to be less infectious, but this may have been underestimated.
A case of herpes zoster on a hospital ward in the USA resulted in three secondary cases of chickenpox among healthcare personnel (1). Two of those infected had not entered the patient's room or been otherwise exposed to the virus. The patient's room had slight negative pressure, but the door opening directly onto the corridor had no airlock. Mapping of air currents revealed a lively exchange of air between the room and the corridor. The authors of the study recommended strict isolation for all patients with varicella zoster virus infection.
A study of 184 patients with herpes zoster in Scotland documented secondary varicella cases in close contacts of 5.4 % of the patients (12). An immunocompromised hospital patient with herpes zoster in England gave rise to 11 cases of chickenpox: five hospital personnel, two fellow patients (one of whom died) and four visitors (13). This underlines the importance of robust containment around immunocompromised patients with herpes zoster, as they are assumed to have larger amounts of virus in their lesions and therefore pose a greater risk of infection.
Covering of herpes zoster lesions has a bearing on the risk of infection. A study from Japan showed that an ordinary gauze dressing easily allowed the virus to pass through, and the virus could be detected on the surface of the dressing and in the room's air filters (2). Occlusive, impermeable hydrocolloid dressings providing full coverage gave no detectable spread of the virus on the surface or in the room. Varicella zoster virus DNA was detected in throat swabs of two out of seven herpes zoster patients. The authors of the study assumed that pharyngeal deposition of the virus occurs on inhalation of particles from the patients' own herpes zoster, and that further transmission from the throat is conceivable. Occlusive, impermeable dressings are recommended when this is anatomically possible.
Reactivation of varicella zoster virus in oral mucosa and droplet transmission is another possible route of infection. In a study of 54 patients with herpes zoster, a polymerase chain reaction test (PCR test) determined that all were positive for varicella zoster virus in saliva early in the course of the disease, and a small number again tested positive after 15 days (14). This reactivation theory is supported by a positive saliva test in one patient in whom incipient herpes zoster was suspected due to the nature of the pain, but who had not yet developed visible lesions. Infectious virus was found in tissue culture in one of two patients when this was tested (14).
Parents who have had chickenpox and are seropositive are occasionally temporary carriers of the virus in nasopharyngeal secretions when their own children have chickenpox (15). The risk of infection that this represents is unknown, but it is appropriate to consider sickness absence if they are healthcare workers.