smallpox vaccine and vaccination in the intensified ... - libdoc.who.int

11. VACCINATION IN THE INTENSIFIED PROGRAMME 559
Table I 1.10. Variation in results of titrations of vaccinia virus according to age of chick embryos at time of
inoculations
Length of
Sample studied
incubation
-I -L,-#.
"I LIIICK
A B C D E
Average
embryo (days' Titreb DlfferenceC Tltreb DlfferenceC Tltreb Dlfferencec Titreb Differencec Titreb Differencec difference
aSource: A. Bernstein (personal communication, 1969).
Expressed as loglo pock-forming units per ml.
C From the titre obtained with i2day-old chlck embryos.
findings, an inoculum of 0.1 m1 and an in- Moisture content
cubati& period of 12 days for the eggs were
acce~ted as standard.
Although residual moisture appeared to be
It was also suggested that the quality of the
an important factor influencing the stability
eggs from different geographical areas might
of freeze-dried vaccine. it was difficult to
affect the assavs. For reasons unknown. vacestablish
criteria for the moisture content.
cines assayed in developing countries often
Sparkes & Fenje (1972) studied the decline
gave titres 0.2-0.3 log unit lower than when
with time in the potency of freeze-dried
tested by the WHO reference centres, a not
vaccine with moisture contents ranging from
unwelcome circumstance. as it ensured with
0.36% to 6.7%, at temperatures of 4 "C,
greater certainty that the vaccine used in the
24 "C, 37 "C and 100 "C (Fig. 11.2). They
field was fully potent.
concluded that a residual moisture content of
To ensure c6m~arabilitv of results. it has
less than 1 was essential for the satisfactory
been decided that assay by pock counting on
storage of freeze-dried smallpox vaccine at
the CA membrane will be used for as long as it
ambient temperatures. ~owever, the test for
is necessary to test samples from the vaccine
moisture content was too expensive and
reserves held in ~eneva and Lausanne (see
technically too difficult to use as a routine,
Chapter 28).
and samples with too high a moisture content
rarely passed the heat-stabilitv test. The
moisture content was sometimes Hssayed with
batches of low heat stability to determine
whether this was due to inadequate freezedrying.
L
I 1
0 4 8 12 16 20 2426
Weeks
Fig. 11.2. Decline with time in titres of samples of
smallpox vaccine of different moisture contents kept
at 24 "C. The relative effects were comparable at
37 "C. but the decline in titre at all moisture contents
higher than 0.36% was greater. (From Sparkes &
Fenje, 1972.)
Improvements in Vaccine Quality
The Intensified Smallpox Eradication Pro-
gramme marked the first time in the history
of WHO that an effective world-wide quality
control programme for biological products
had been established (Fig. 11.3). Between
1967 and 1980,2578 production batches were
tested, the annual number of samples ranging
from 392 in 1973 to 46 in 1980 (Table 11.1 1).
From 1969 onwards there was a considerable
improvement in vaccine quality. Between
1967 and 1972 initial potency was unsatisfac-
tory in 47% of instances, heat stability in