Medical Microbiology

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.,

7

Kayser, Medical Microbiology © 2005 Thieme