JAPANESE ENCEPHALITIS VIRUS VACCINE

5 Jun

Japanese encephalitis (JE) is a mosquito-born infection endemic to parts of Asia.  The  flaviviral neurologic infection is closely related to  St.  Louis  encephalitis and  West  Nile  virus. This infection causes an  average of 35,000 reported  cases and 10,000 deaths each  year (209), although the  majority of infec- tions are  subclinical. Viremia develops after a  bite  from  an infected mosquito and  1 out  of 250  infections leads to symp- tomatic disease (211).  Most  infections clear before  the  virus enters the  central nervous system. However, once  neurologic invasion occurs, large areas of the  brain may  be involved. The resulting encephalitis is typically severe, with a 25–40%  fatal- ity rate (212,213). Residual neurologic sequelae are  evident in 10–30%  of cases (212). Japanese encephalitis is seasonal with most  cases occurring after infection during the  rainy season; in temperate areas, this is from  June through September. In the  more  tropical areas, the  season begins in  March and extends until October.

Several findings related to JE infection are:

1.   Poorer performance on standardized tests (compared with uninfected subjects).

2.   Those   who  had   dengue fever   infection earlier  may have decreased morbidity and mortality rates, possibly due to the  presence of other antiflavivirus antibodies.

3.   Risk factors for death include documented virus in CSF, low levels  of IgG or IgM, and  decreased sensorium.

Control of vectors and   reservoirs of infection aid  in decreasing cases of JE. These measures are: 1) control of mos- quitoes and  avoidance of areas where mosquitoes are  likely  to occur; 2) draining or spraying of swamps and  other areas with standing water; 3) humans and  other mammals may  be dead- end  hosts requiring no containment; and  4) agricultural ani- mals (pigs)  and  endemic birds (egrets and  herons) may  be amplifying hosts with high-grade viremia.

Three vaccines are available worldwide. The one used com- mercially for travelers is derived from mouse brain and is a form- aldehyde inactivated  vaccine. The  vaccine contains a Beijing-1

strain, thimersol, gelatin, and  other components. The  vaccine is administered as 3 doses  on days  0, 7, and  30.  More frequent inoculations may  be given  (5–7  days  apart) when there is a need  for a quick  immunization schedule, although antibody response is  lower  and  may  not  last as  long.  The  vaccine is licensed for  persons >1  year of age  in  the  United States (214,215).  The  vaccine is recommended for travelers to Asia who will be spending a month or longer in endemic areas dur- ing the  transmission season of the  virus (which varies accord- ing  to  geographic region) (213).  Two  other JE vaccines are licensed in  China: an  inactivated JE vaccine derived from hamster kidney and  a live attenuated vaccine from the  same source combined with the  SA14-14-2 viral strain. The  latter is less  costly  and  is replacing the  inactivated virus vaccine. The  efficacy  record of this vaccine is reported to be greater (Table 4.6).

Adverse Effects

Systemic side effects. Fever, headache, malaise, chills, dizziness, rash, myalgia, abdominal pain, and  nausea and  vomiting are  reported.

Adverse neurologic events. Encephalitis, peripheral neuropathy, or other adverse neurologic events occur in

1.0 to 2.3 cases per  1 million vaccinations (216).

Allergic mucocutaneous reactions. The  mouse brain vaccine has  been  associated with 73 allergic mucocuta- neous reactions (217).

Adverse allergic reaction. May  occur  within minutes or as late as 17 days  after vaccination; most  occur with- in 48 hours. Those  with a history of allergic rhinitis or urticaria  development (insect stings or  bites) have a great risk (218,219).

General. 1 of 260 vaccinees complains of a general rash, itching, or swelling, especially in the  areas of the  face, lips, and  throat, and/or the  extremities.

Special Considerations

Travelers. A 10-day period following vaccination is rec- ommended before  traveling due  to  possibility of ad- verse events.

Live virus vaccines. Should live virus vaccines (such  as MMR) be necessary, it is better to administer two doses of JE vaccines before  the  live virus vaccines for maxi- mum efficacy.

Malaria. The  efficacy  of JE vaccine is lessened if chloro- quine is being  taken for prophylaxis against malaria.

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