Medical Microbiology

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Prevention 403

Therearetwobasictypesofvaccines:
Activeimmunization.Inthismethod,theantigen(virus)isintroducedinto
thebody,eitherinaninactivatedform,orwithattenuatedpathogenicitybut
stillcapableofreplication,toenablethebodytobuildupitsownimmunity.
&Inactivatedvaccines.Theimmunitythatdevelopsafterso-called“dead
vaccines”areadministeredismerelyhumoralandgenerallydoesnotlast
long.Forthisreason,boostervaccinationsmustbegivenrepeatedly.The
mostimportantdeadvaccinesstillinusetodayareinfluenza,rabies,some
flavivirus,andhepatitisAandBvaccines.Someinactivatedvaccinescontain
themostimportantimmunogenicproteinsofthevirus.Theseso-calledsplit
vaccinesinducemoreefficientprotectionand,aboveall,arebettertolerated.
Someofthemarenowproducedbygeneticengineeringmethods.
&Liveattenuatedvaccines.Thesevaccinesconfereffectiveandlong-last-
ingprotectionafteronlyasingledose,becausethevirusescontainedinthem
arecapableofreplicationinthebody,inducingnotonlyhumoral,butsome-
timescellularimmunityaswell,nottomentionlocalimmunity(portalof
entry!).Suchlivevaccinesarepreferablewhenavailable.Thereare,however,
alsodrawbacksandrisks,amongthemstability,theincreasedpotentialfor
contaminationwithotherviruses,resultinginmorestringenttestingandthe
possibilitythataback-mutationcouldproduceapathogenicstrain(seeVaria-
bilityandQuasispeciesofViruses,p. 39 1).
&Vaccineswithrecombinantviruses.Sinceonlyasmallnumberof(sur-
face)viralproteinsarerequiredtoinduceprotectiveimmunization,viralvec-
torsareusedinattemptstoexpresstheminvaccinerecipients(seep.390).
Suitablevectorsincludetheleastvirulentvirusstrainsamongthepicorna-
viruses,alphaviruses,andpoxviruses.Theremustbenogeneralizedimmu-
nitytothevectorinthepopulationsothatitcanreplicateinvaccinerecipi-
entsandthedesiredproteinwillatthesametimebeexpressed.Suchrecom-
binantvaccineshavenotyetbeenapprovedforuseinhumans.Arabiesvac-
cinecontainingtherecombinantvacciniavirusforuseinanimalsistheonly
practicalapplicationofthistypesofar(p.390).
&NakedDNAvaccine.SincepureDNAcanbeinsertedintoeukaryoticcells
(transfection)andtheinformationitcarriescanbeexpressed,DNAthatcodes
forthedesired(viral)proteinscanbeusedasvaccinematerial.Theadvan-
tagesofsuchvaccines,nowstillinthetrialphase,includeeaseofproduction
andhighstability.
Passiveimmunization.Thistypeofvaccineinvolvestheinjectionofantibod-
iesusingonlyhumanimmunoglobulins.Theprotectionconferredisofshort
durationandonlyeffectiveagainstvirusesthatcauseviremia.Passiveimmu-
nizationisusuallyadministeredasapostexposureprophylacticmeasure,i.e.,

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Kayser, Medical Microbiology © 2005 Thieme

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