4.4. Immunity period
It should be noted that in order to achieve a successful vaccination,
the development of associated antibody and cellular immune responses
against SARS-CoV2 should be sustained for a long time. The SARS-CoV2
spends a limited time in our communities, and it is too early to comment
on the longevity of induced protective immune responses with high
certainty. Although, it is possible to somewhat predict the quality and
longevity of antibody and T cell responses to the COVID-19 vaccine
candidates by inspiration from vaccine studies for two closely related
coronaviruses, MERS and SARS , which have provided promising
long-lasting protective immune responses [33, 34].
4.5. Disease enhancement
phenomenon
The biggest challenge is that not only the designed vaccine award
immunity against the desired infectious agent but also aggravates the
course of the disease and enhanced mortality [34, 35]. disease
enhancement or antibody-dependent enhancement (ADE) is such a wrecking
process that mediated by non-neutralizing antibody responses against
vaccine candidates. This phenomenon aborts the vaccine project by
vitiating the elementary vaccination goal and making the disease worse.
Indeed, the ADE is mediated by Fc receptor or complement coated cells
that following antibody attachment, reverted immune responses from Th1
(interleukin (IL)-2, tumor necrosis factor alpha (TNF-α), interferon
gamma (IFN-γ)) toward Th2 responses (IL-10, IL-6, prostaglandin (PGE2),
IFN-α) and blocking signal transducer and activator of transcription
(STAT) signaling pathways leading to unbridled viral replication
[27]. So, regarding the ADE as the major bane of successful
vaccination, maximum efforts should be done to identify efficient
immunodominant epitopes and prevent the development of dysfunctional
antibodies responsible for the disease exacerbation.