With 79 vaccines currently under development worldwide to combat COVID-19, researchers are trying to obviate a potential but dangerous obstacle: immune enhancement.
Immune enhancement is a strange phenomenon where immunity to a particular virus can exacerbate an infection instead of preventing it.
Such an immune reaction was reported with a vaccine against dengue fever, caused by a flavivirus — another family of viruses. Clinical trial data from Dengvaxia, a recently approved vaccine for the mosquito-borne infection, showed a higher rate of hospitalisations for dengue three years after vaccination in young children, compared to children who were unvaccinated.
The increased risk appears to be for children who had had no previous dengue infections when they were vaccinated.
There are four types of dengue viruses, each of which are different. Immunity to one of the viruses does not protect against the others. On the contrary, it increases the risk of having severe illness with a second infection.
WHO experts recommended that countries that want to use the vaccine should ideally limit its use to places where at least 70 percent of would-be recipients have had at least one dengue infection already.
Tests of vaccine candidates for respiratory syncytial virus (RSV) and SARS have shown similar immune enhancement.
This paradoxical phenomenon may manifest either as antibody-dependent enhancement (ADE) or cell-based enhancement. The former is a process in which a virus leverages antibodies to aid infection whereas the latter category includes allergic inflammation caused by Th2 immunopathology.
Some epidemiologists believe the bigger problem with the COVID-19 virus is probably Th2 immunopathology, even though
there is potential for ADE.
However, considering the wide range of approaches that are currently underway to develop a successful vaccine against the novel coronavirus, researchers may eventually overcome this challenge.