CLINICAL PHARMACOLOGY
Rabies In The United States
Over the last 100 years, the epidemiology of rabies in
animals in the US has changed dramatically. More than 90% of all animal rabies
cases reported annually to the Centers for Disease Control and Prevention (CDC)
now occur in wildlife, whereas before 1960 the majority was in domestic
animals. The principal rabies hosts today are wild terrestrial carnivores and
bats. Annual human deaths have fallen from more than a hundred at the turn of
the century to 1 to 2 per year despite major epizootics of animal rabies in
several geographic areas. Within the US, only Hawaii has remained rabies free.
Although rabies among humans is rare in the US, every year tens of thousands of
people receive rabies vaccine for postexposure prophylaxis.
Rabies is a viral infection transmitted via the saliva of
infected mammals. The virus enters the central nervous system of the host,
causing an encephalomyelitis that is almost invariably fatal. The incubation
period varies between 5 days and several years, but is usually between 20 and
60 days. Clinical rabies presents either in a furious or in a paralytic form.
Clinical illness most often starts with prodromal complaints of malaise,
anorexia, fatigue, headache, and fever followed by pain or paresthesia at the
site of exposure. Anxiety, agitation, and irritability may be prominent during
this period, followed by hyperactivity; disorientation; seizures; aerophobia
and hydrophobia; hypersalivation; and eventually paralysis, coma, and death.
Modern day prophylaxis has proven nearly 100% successful;
most human fatalities now occur in people who fail to seek medical treatment,
usually because they do not recognize a risk in the animal contact leading to
the infection. Inappropriate postexposure prophylaxis may also result in clinical
rabies. Survival after clinical rabies is extremely rare, and is associated
with severe brain damage and permanent disability.
RabAvert (in combination with passive immunization with
Human Rabies Immune Globulin [HRIG] and local wound treatment) in postexposure
treatment against rabies has been shown to protect patients of all age groups
from rabies, when the vaccine was administered according to CDC's Advisory
Committee on Immunization Practices (ACIP) or World Health Organization (WHO)
guidelines and as soon as possible after rabid animal contact. Anti-rabies
antibody titers after immunization have been shown to reach levels well above
the minimum antibody titer accepted as seroconversion (protective titer) within
14 days after initiating the postexposure treatment series. The minimum
antibody titer accepted as seroconversion is a 1:5 titer (complete inhibition
in the rapid fluorescent focus inhibition test [RFFIT] at 1:5 dilution) as
specified by CDC1 or ≥0.5 IU/mL as specified by WHO.2,3
Clinical Studies
Preexposure Vaccination
The immunogenicity of RabAvert was demonstrated in
clinical trials conducted in different countries such as the US,4,5 the
United Kingdom (UK),6 Croatia,7 and Thailand.8-10
When administered according to the recommended immunization schedule (Days 0,
7, and 21 or 0, 7, and 28), 100% of subjects attained a protective titer. In 2
studies carried out in the US in 101 subjects, antibody titers >0.5 IU/mL
were obtained by Day 28 in all subjects. In studies carried out in Thailand in
22 subjects and in Croatia in 25 subjects, antibody titers of >0.5 IU/mL
were obtained by Day 14 (injections on Days 0, 7, and 21) in all subjects.
The ability of RabAvert to boost previously immunized
subjects was evaluated in 3 clinical trials. In the Thailand study, preexposure
booster doses were administered to 10 individuals. Antibody titers of >0.5
IU/mL were present at baseline on Day 0 in all subjects.9 Titers
after a booster dose were enhanced from geometric mean titers (GMTs) of 1.91 to
23.66 IU/mL on Day 30. In an additional booster study, individuals known to
have been immunized with Human Diploid Cell Vaccine (HDCV) were boosted with
RabAvert. In this study, a booster response was observed on Day 14 for all
individuals (22/22).11 In a trial carried out in the US,4
an IM booster dose of RabAvert resulted in a significant increase in titers in
all subjects (35/35), regardless of whether they had received RabAvert or HDCV
as the primary vaccine.
Persistence of antibody after immunization with RabAvert
was evaluated. In a trial performed in the UK, neutralizing antibody titers
>0.5 IU/mL were present 2 years after immunization in all sera (6/6) tested.
Preexposure Vaccination In Children
Preexposure administration of RabAvert in 11 Thai
children aged 2 years and older resulted in antibody levels higher than 0.5
IU/mL on Day 14 in all children.12
Postexposure Treatment
RabAvert, when used in the recommended postexposure WHO
program of 5 to 6 IM injections of 1 mL (Days 0, 3, 7, 14, and 30 and
optionally on Day 90) provided protective titers of neutralizing antibody
(>0.5 IU/mL) in 158/160 patients8,9,13-16 within 14 days and in
215/216 patients by Days 28 to 38.
Of these, 203 were followed for at least 10 months. No
case of rabies was observed.8,9,13-20 Some patients received HRIG,
20 to 30 IU/kg body weight, or Equine Rabies Immune Globulin (ERIG), 40 IU/kg
body weight, at the time of the first dose. In most studies,8,9,13,17
the addition of either HRIG or ERIG caused a slight decrease in GMTs which was
neither clinically relevant nor statistically significant. In one study,16 patients
receiving HRIG had significantly lower (P<0.05) GMTs on Day 14; however,
this was not clinically relevant. After Day 14 there was no statistical significance.
The results of several studies of normal volunteers
receiving the postexposure WHO regimen, i.e., “simulated” postexposure, showed
that with sampling by Days 28 to 30, 205/208 vaccinees had protective titers
>0.5 IU/mL.
No postexposure vaccine failures have occurred in the US
since cell culture vaccines have been routinely used.1 Failures have
occurred abroad, almost always after deviation from the recommended
postexposure treatment protocol.21-24 In 2 cases with bites to the
face, treatment failed although no deviation from the recommended postexposure
treatment protocol appeared to have occurred.25
Postexposure Treatment In Children
In a 10-year serosurveillance study, RabAvert was
administered to 91 children aged 1 to 5 years and 436 children and adolescents
aged 6 to 20 years.19 The vaccine was effective in both age groups.
None of these patients developed rabies.
One newborn received RabAvert on an immunization schedule
of Days 0, 3, 7, 14, and 30; the antibody concentration on Day 37 was 2.34
IU/mL. There were no clinically significant adverse events.26
REFERENCES
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Morbidity and Mortality Weekly Report Recommendations and Report, January 8,
1999, Vol.48, RR-1; 1.1-21.
2. Smith JS, Yager, PA & Baer, GM. A rapid
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3. Eighth Report of the WHO Expert Committee on Rabies.
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for preexposure immunization. Vaccine. 1989; 7: 397-400.
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