Protective effect of immediate inoculation of a live varicella vaccine in household contacts in relation to the viral dose and interval between exposure and vaccination.

The protective effect on 45 household contacts of immediate inoculation of a live varicella vaccine was examined in relation to the viral dose and days after exposure to varicella. The viral doses used were 300 PFU to 15,000 PFU, and the intervals between exposure and vaccination were 0 to 5 days. All 30 contacts given viral doses of 800 to 15,000 PFU within 3 days after exposure were protected from clinical varicella. With lower viral doses of 300 to 600 PFU within 3 days after exposure, 6 to 10 contacts developed rashes 13 to 25 days after vaccination, but all showed mild symptoms. When vaccine was given 5 days after exposure, even with a dose of 1,500 to 4,000 PFU, typical varicella symptoms appeared 6 to 9 days after vaccination. These results suggest that vaccination of household contacts with viral doses of over 500-800 PFU within 3 days after exposure is effective in preventing manifestation of clinical varicella and that clinical symptoms may be modified to a mild form by vaccination with even a lower viral dose when given within 3 days after exposure.