To reduce reactogenicity, most of the new vaccines under development are based on well-defined molecular immunogens, as opposed
to whole attenuated or inactivated pathogens. These "molecular vaccines" encompass proteins, peptides, lipopeptides, plasmid
DNA, or recombinant viruses based on viral vectors know to be safe to humans. Such vaccines are generally not as immunogenic
as traditional vaccines and require adjuvants to induce a more potent and persistent immune response. New vaccine targets
often require induction of strong cellular responses, including T helper cells and sometimes cytotoxic T lymphocytes in addition
to antibodies. Conventional adjuvants based on aluminum salts predominately induce antibody responses.4–5
Discovering new adjuvants is crucial for the development of vaccines that require cell–mediated responses. The recent shift
from an empirical to a rational approach to adjuvant development was made possible by an increased understanding of the control
mechanism in the immune system, and of the interplay between the innate and the acquired immune response. In particular, the
role of toll-like receptors (TLRs) in recognizing pathogen-associated molecular patterns and the ability to stimulate these
receptors using a range of new agonists of varying specificities has significantly advanced the adjuvant field.6
Several TLR agonists have been studied as vaccine adjuvants in clinical studies. The main feature shared by all TLR agonists
is their ability to deliver a potent activation signal to antigen presenting cells. CpG oligodeoxynucleotide (ODN), a TLR9
agonist, was shown to be a potent adjuvant of both humoral and cell-mediated immunity in human studies. Monophosphoryl lipid
A (MPL), a TLR4 agonist, has been shown to induce both antibody and T-cell responses. It was licensed as an adjuvant for hepatitis
B vaccine nonresponders in Europe in 2005.
NEW VACCINES BEING DEVELOPED
Great progress has been made toward the development of vaccines against emerging infectious pathogens (e.g., HIV and HSV);
cancers; diabetes; rheumatoid arthritis; multiple sclerosis; Alzheimer disease; allergies to either tree pollen, grass pollen,
or house dust mites; hypertension; and cholesterol management. Some of these are highlighted below.
Flu Vaccines—Seasonal and Pandemic
The currently licensed flu vaccines are trivalent inactivated virus particles containing at least 15 μg of hemagglutinin polypeptide
from each of the three strains selected for that year. They are manufactured using chicken eggs and have several disadvantages:
insufficient supply because of limited manufacturing capability, long lead time (>6 months) from identification of strains
to start of vaccine distribution, a cold-chain requirement for distribution, short shelf-life (cannot be stockpiled), and
needle delivery. Additionally, healthy eggs may not be available due to avian flu.
Cell culture based vaccines are under active development and have several potential advantages: greater controllability, higher
yield, faster manufacturing, and no reliance on chicken eggs.7 The first cell culture based flu vaccine (Optaflu) was approved in Europe in June 2007.
Another area of active research to improve current flu vaccines is antigen-sparing through the use of adjuvants and more efficient
deliveries. The use of aluminum adjuvant has been shown to reduce the amount of antigens required by at least two-fold.8 An influenza vaccine with an oil-in-water emulsion adjuvant (MF59) is licensed in Europe.9 The co-administration of flu vaccines with GM-CSF or IL-2 in lipid vesicles also has been investigated.10
DNA-based flu vaccines expressing M2 and NP influenza proteins are currently under development. They have the advantages of
rapid manufacturing in high volumes (due to a short lead time), long shelf -life (so they can be stockpiled), no need for
cold-chain distribution, and possible needle-free delivery.