Experts revive interest in vaccine for West Nile virus amid ongoing threat
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Key takeaways:
- The biggest West Nile virus outbreak in U.S. history occurred in 2021 in Arizona.
- There are no vaccines to prevent West Nile virus infection and no medications to treat it.
CDC experts argued recently that the time is right to revisit the need for a vaccine against West Nile virus, which remains the leading cause of mosquito-borne disease in the United States.
The biggest localized West Nile virus outbreak in U.S. history occurred in 2021 in three counties in Arizona, including Maricopa County, where Phoenix is located. The 1,140 cases in Arizona represented more than half of the almost 3,000 cases reported in the U.S. that year, according to a study published last week in MMWR.
West Nile virus was first detected in the U.S. in 1999 and is thought to have arrived via an infected animal, most likely a bird. There are no vaccines to protect people from infection with the virus, and no medications to treat it.
“West Nile virus has been endemic in the United States for 20 years,” J. Erin Staples, MD, PhD, a medical epidemiologist at the CDC, told Healio.
“Our current prevention messages” — using insect repellent and wearing long-sleeved shirts and long pants — “are not really enough to reduce the ongoing burden,” Staples said. “A West Nile vaccine is really what we think at the CDC would be most effective at preventing West Nile-related disease and deaths.”
Staples and colleagues earlier this year published a perspective in The New England Journal of Medicine arguing for a resurgence in West Nile virus vaccine research
Since 1999, there have been more than 55,000 reported West Nile viruses infections in humans in the U.S., including more than 27,000 cases of neuroinvasive disease and 2,600 deaths, according to Staples and colleagues.
According to the MMWR report, there were 2,911 cases reported in the U.S. in 2021 — the most since 2021. (CDC tracking shows 1,126 cases in 2022 and 455 cases reported so far in 2023.)
Among the cases reported in 2021, 69% were neuroinvasive, and 71% of patients reported illness onset between July and September. The median patient age was 65 years, and 60% of patients were male. Overall, 72% were hospitalized and 8% died. The median age of those who died was 75 years.
The rate of neuroinvasive West Nile virus disease in 2021 — 0.61 cases per 100,000 population — was the highest it had been in the U.S. since 2012, and nearly double the median rate observed from 2010 to 2020.
The size of West Nile virus outbreaks remains unpredictable, Staples said. She pointed to the 2021 outbreak in Arizona, where typically there may be dozens or hundreds of cases, but not thousands.
“We don’t know exactly when or where those explosive outbreaks will occur, but we know they will occur every year,” Staples said.
She said the CDC runs a challenge for academic centers to make annual predictions for potential outbreaks, which has “yet to find a Holy Grail method for prediction.”
“From our prediction challenge, we know that one of the biggest predictors is if you’ve seen West Nile virus cases in a place before, you’re likely to see them again,” Staples said. “So, if you’ve had West Nile where you live, you should be paying attention every summer when those mosquitoes get active.”
Several West Nile vaccine candidates have demonstrated safety and immunogenicity in phase 1 and 2 trials, she said, but a large phase 3 trial — which could demonstrate clinical efficacy — is challenging to pull off because the unpredictability of West Nile virus outbreaks makes it difficult to choose locations for the trials.
According to Staples, the CDC has considered several models for potential phase 3 studies, including using an animal model, with the intention to collect data later on human efficacy as people receive the vaccine.
Other pathways have been ruled out, such as the use of human challenge studies, in which study participants are intentionally infected with the pathogen. West Nile virus is too dangerous for that approach, Staples and colleagues said, which is why a large trial would have to take place in the context of an actual outbreak.
Staples said targeting at-risk people in locations with a high incidence of West Nile virus —rather than rolling out a national vaccination campaign, for example — could decrease the national burden of West Nile virus by 60% and reduced deaths by 30%.
“Having another tool in our toolbox like a vaccine to prevent people from getting and dying from West Nile virus is very important to us,” she said.
References:
- CDC. West Nile virus: Current year data (2023). https://www.cdc.gov/westnile/statsmaps/current-season-data.html. Accessed Aug. 30, 2023.
- CDC. West Nile virus: Historic data (1999-2022). https://www.cdc.gov/westnile/statsmaps/historic-data.html. Accessed Aug. 31, 2023.
- Fagre AC, et al. Morb Mortal Wkly Rep. 2023;doi:10.15585/mmwr.mm7234a1.
- Gould CV, et al. N Engl J Med. 2023;doi:10.1056/NEJMp2301816.
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