Stability of diluted smallpox vaccine under simulated clinical conditions.
BACKGROUND
During a mass smallpox immunization campaign, vaccine may be exposed to ambient temperatures for extended periods of time.


OBJECTIVE
To determine the viability of undiluted and 5x diluted DryVax smallpox vaccine after cycling vaccine in and out of refrigeration for 2 weeks, as might occur during an immunization campaign.


METHODS
Two vials of Dryvax vaccine were reconstituted as per manufacturer's instructions (1x) and two vials were reconstituted using 5x the recommended diluent (5x). Every 12h over 2 weeks, vials were cycled between refrigeration and room temperature (1x-RT, 5x-RT) or ice bath (1x-cold, 5x-cold). Each vial was sampled in triplicate at time of reconstitution and thereafter at 24 or 48 h intervals.


METHODS
Viability measured by viral plaque forming units per ml (pfu/ml).


RESULTS
All four vaccine vials showed a decline in virus titer over the 2-week period but remained well above 10(7)pfu/ml. Compared with titers on the day of reconstitution (day 0), titers at the end of the study (day 14) had declined by 0.4--0.6l og in all vials (Table 1). Linear regression analysis suggested that decay in viral titer occurred more rapidly in vials exposed to room temperature compared with vials kept on ice and in vaccine diluted 1x compared with vaccine diluted 5x.


CONCLUSIONS
After 2 weeks, viability was greater than 10(7)pfu/ml, the titer suggested by Frey et al. as necessary to ensure successful vaccination in more than 97% of vaccinees. When removed from refrigeration, keeping the vaccine on ice lowers the decline in titer.
Kline
Richard L
RL
National Center for Infectious Diseases, Centers for Disease Control and Prevention MS G-43 1600 Clifton Rd NE Atlanta, GA 30333, USA.
Regnery
Russell L
RL
Armstrong
Gregory L
GL
Damon
Inger K
IK
D004355:Drug Stability; D004356:Drug Storage; D012034:Refrigeration; D012900:Smallpox Vaccine; D013696:Temperature; D014616:Vaccinia virus / Q000302:isolation & purification; D010948:Viral Plaque Assay
C078786:DryVax vaccine; D012900:Smallpox Vaccine