INTRODUCTION: Varicella-zoster virus (VZV) is a highly contagious Herpesvirus which causes chickenpox (primary infection) or shingles (upon reactivation). VZV poses a nosocomial infection for healthcare workers and patients. The incubation period for the virus is between 10-21 days. It is generally thought that patients are infectious 1-2 days prior to rash onset and between 4-7 days post rash onset. However, there is a lack of data showing the route of transmission, other than from direct contact.
DISCUSSION: To better understand the route and timing of VZV transmission, Marin et al1., reviewed the medical literature to assess the routes of transmission prior to rash onset. They searched Medline, Embase, Cochrane Library and CINAHL databases for articles published (any language) through October 31 of 2019. The search terms were ‘Varicella’, ‘chickenpox’, or ‘herpes zoster’, along with ‘transmission’ or ‘spread’ or ‘epidemic’ or ‘isolation’ as well as ‘respiratory’ or ‘airborne’ or ‘nasal/pharyngeal/throat swab’ or ‘before rash’. They screened 693 articles and identified 59 for full-text review. The articles were from around the world, including Europe, Asia, and the United States. Seven articles discussed VZV transmission before varicella rash onset using epidemiological data and 10 with laboratory data. Three of these articles included data from reports of varicella cases during outbreaks in hospitals, three included institutional outbreaks (schools or one in a jail). Each report indicated transmission from an index patient who exposed contacts before being diagnosed with varicella. The reports showed that patients with varicella were infectious at various intervals before the rash appeared; <1 day before in one report, at least 1 day before in two papers, at least 2 days prior in two papers, and at least 4 days prior in one paper. Of the 10 articles that addressed VZV transmission before rash onset with laboratory evidence, five were published during 1966-1989 using the virus culture to identify virus in the oropharynx. Five articles from 1991-1999 examined evidence of VZV DNA presence in the oropharynx using polymerase chain reaction (PCR). Four studies included exposed siblings and three included exposed patients, one study included exposed daycare contacts and one included children and adult participants in a clinical trial for acyclovir after the appearance of skin lesions. This review of literature confirms the scarcity of evidence in the medical literature on transmission of VZV before varicella rash onset. Several outbreak investigations in healthcare facilities reported potential exposure prior to rash onset in a patient with varicella but no transmission among exposed staff and patients. Laboratory evidence supporting transmission of VZV before rash onset is also very limited. VZV DNA was identified by PCR during the incubation period in only one study. However, methods for specimen collection and storage varied among the studies and may have been suboptimal. In addition, the presence of VZV DNA does not necessarily indicate infectivity or transmissible virus. Isolation of VZV in the respiratory tract of varicella patients is rarely reported. Coughing/sneezing leading to VZV transmission is not characteristic of varicella.
CONCLUSION: Based on the available medical literature, VZV transmission seems unlikely prior to rash onset. The authors suggest that providers should continue to implement appropriate infection control measures since the possibility of pre-rash, respiratory transmission of VZV cannot be entirely ruled out. Additional evidence using current or newly developed laboratory VZV assays would be beneficial.
REFERENCES:
- Marin, M., J. Leung, A.S. Lopez, L. Shepersky, D.S. Schmid, and A. A. Gerson. (2021). Communicability of varicella before rash onset” a literature review. Epidemiology and Infection 149, e131, 1-7. https://doi.org/10.1017/S0950268821001102
MKTG 1065 - Rev A 020322
