Zika cases likely to continue in US; vaccine foreseeable in near future
Infectious Diseases in Children asked Editorial Board member H. Cody Meissner, MD, professor of pediatrics at Tufts University School of Medicine and director of pediatric infectious disease at Tufts University, to discuss obstacles in preventing the spread of Zika virus, and if he believes there will be an effective Zika vaccination implemented in the near future.
In the absence of mediation or a vaccine to treat Zika virus infections in endemic areas, the best advice is to prevent infection by avoiding mosquito bites, eliminating areas where mosquitos breed, enhancing mosquito control and using precaution to prevent sexual transmission between humans. Other known routes of infection include blood transfusion or organ transplantation. Most worrisome is transplacental transmission during pregnancy from an infected mother to her unborn infant.
Zika is just the most recent example of the difficulties encountered when trying to control a new infectious disease when it suddenly emerges. We have seen this with other novel infectious diseases such as severe acute respiratory syndrome, Middle East respiratory syndrome, Ebola virus, enterovirus D-68, West Nile virus and the 2009 pandemic influenza.
Zika virus is a flavivirus first detected in the Zika Forest of Uganda in 1947. The first documented outbreak of human disease occurred in 2007 in the Federated States of Micronesia. Zika first appeared in the Western Hemisphere in the summer of 2015 in Brazil and subsequently spread throughout much of the Americas. The first full season in 2016 saw a high burden of disease in South America. In 2017, the amount of disease was substantially lower in Brazil and the surrounding countries. One unanswered question is will there be enough “herd immunity” from the infection itself so that each successive year, the wave or burden of disease will become smaller and smaller? Zika virus is spread primarily by the bite of an Aedes mosquito, mainly the A. aegypti mosquito. The A. aegypti species is found mostly in the southern states. The virus is less frequently transmitted by the A. albopictus mosquito, which may extend as far west as California, as far north as the Ohio valley and into the plains states. However, the A. albopictus mosquito is not as effective in transmitting the infection as A. aegypti. In the United States, Zika cases have occurred in several states including Florida and Texas, and new cases are likely to be reported in this country, even among non-travelers.
In endemic areas, such as Brazil and other countries in Central and South America, once a sufficient number of people develop immunity from either the infection or from a vaccine, burden of disease will decline. About 80% of infected people do not develop symptoms and experience only asymptomatic seroconversion.
It is likely that a safe and effective Zika virus vaccine will be available in the near future.
Three approaches are being explored to develop a vaccine. The first is the traditional route by which many vaccines are made. Large amounts of virus are grown in grown in cell culture and then inactivated by use of either a chemical or heat so that the virus is no longer infectious. The attenuated strain retains the ability to stimulate a protective immune response in the vaccinated person.
Two other approaches to vaccine development are being evaluated. One is to inject DNA encoding for a viral protein that is responsible for stimulating neutralizing antibodies. This can be achieved by an intramuscular injection of either DNA or by injecting the appropriate messenger RNA into the human muscle cell where the viral protein is synthesized. When the immune response encounters that viral protein, protective antibodies are produced.
The third approach utilizes vector vaccines. The gene(s) from the Zika virus that are responsible for making viral proteins that induce a neutralizing antibody response are inserted into a viral vector such as an adenovirus. By stitching these extra Zika virus genes into the adenovirus and then injecting the modified adenovirus, when the adenovirus replicates copies of the Zika virus, protein(s) are produced, resulting in antibody production.
Because of the extensive effort from government, academic and industry sources, it is likely that a vaccine will soon be available.
Disclosure: Meissner reports no relevant financial disclosures.