cell resulting in entry of the virus. Therefore, instead of neutralizing the virus, the

antibody enhances the infection by increasing viral entry into target cells [9].

3.4.2

LIVE ATTENUATED VIRUS

There are several ways to attenuate a virus. One of these is to select a variant that

cannot infect human cells as efficiently as the wild-type strain. This can be ac-

complished by passaging a pathogenic virus in a cell type with a different origin

(such as monkey cells). At first, it will not be able to infect the monkey cells very

well, but with increasing passages, variants with higher infectivity for the monkey

cells, will be selected, such that, at one point, the variant that emerges will be

weakened in its ability to infect human cells. Thus, this virus although still be able

to replicate, cannot enter human cells as easily, and is therefore attenuated in its

ability to cause disease in humans. Similarly, the virus can be adapted for survival at

lower temperatures, making the human body an inhospitable host [10].

More recently, genetic strategies of inactivation have been developed. These

include altering post-translational processing, the use of rare codon pairs or inter-

fering with the virus’s ability to suppress anti-viral pathways in the host [11].

A major advantage of using a live attenuated virus for vaccination is that it elicits the

most robust immunity. However, since it can cause mild disease (because there is a

replicating virus), it cannot be used for immunocompromised individuals, since they

may suffer from more severe symptoms in the absence of an adequate immune response.

This method has been used successfully for yellow fever vaccines. In fact, the

yellow fever virus has also been used as a vector developing vaccines for other

viruses. For instance, if dengue virus genes introduced in yellow fever virus, an

immune response will be generated to the Dengue virus also [12].

3.4.3

INACTIVATED VIRUS

To increase safety, pathogens can be inactivated so that they cannot replicate. This

can be achieved by heat, by chemicals, or by radiation and although the virus is

inactive, it maintains its immunogenicity, so that the immune system can still re-

cognize it and develop an immune response.

Vaccine preparations of inactivated pathogens have several advantages: in ad-

dition to being more stable than those of attenuated viruses, these vaccines can also

be administered to immunocompromised individuals. The disadvantage, however,

is that the vaccine is not as strong and therefore needs a booster [10].

3.4.4

SUBUNIT VACCINE

When a part of the pathogen is used as the immunogen, it is referred to as a subunit

vaccine. Sub-unit vaccines are often proteins or their fragments, but can also be

polysaccharides, such as those making up the bacterial capsule [10].

Given that only antigens corresponding to one specific component and not the

whole virus, are presented to the immune system, sub-unit vaccines are not as

efficient as inactivated or attenuated vaccines. Therefore, careful consideration has

Introduction to basic immunology

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