Jan. 16 (UPI) -- Creating the annual flu vaccine may not be an exact science, but drugmakers working in tandem with epidemiologists around the world seem to get it right more often than not.
Four strains of the influenza virus have been identified through lab tests as circulating across the United States so far this flu season.
The vaccines approved for use this year cover at least two of them, officials say, but they won't know exactly how effective the shot has been until the season is over.
"There is some concern about different kinds of vaccine mismatch, but it is hard to know in real time how effective the vaccine is," Tony Moody, an associate professor of pediatric infectious diseases and immunology at Duke University Medical Center, told UPI. Moody is one of the principal investigators for Duke's Collaborative Influenza Vaccine Innovation Centers, where he works to identify potential influenza vaccine candidates.
"Regardless, we know that even in years when the vaccine is only partially effective, it is still more effective than not getting the vaccine -- so we recommend that people get the current vaccine, even if it is not perfect," he said.
According to data from the U.S. Centers for Disease Control and Prevention, over the past decade, the approved vaccines -- which are developed for each flu season by private manufacturers -- have been at least 40 percent effective at preventing hospitalizations from lab-confirmed cases of influenza. At their best, though, the shots have been 60 percent effective.
Notably, vaccine effectiveness has declined in each of the past four flu seasons for which data are available: from 48 percent in 2015-16 to 40 percent in 2016-17, 38 percent effective in 2017-18 and 29 percent effective in 2018-19. Preliminary data on the effectiveness of this year's shot will be available in February, the CDC said.
"A flu vaccine is the first and best defense against influenza, and CDC recommends ongoing vaccination as long as influenza viruses are circulating," a representative of the agency said. "Flu activity is typically highest between December and February, although it can last as late as May. Even if we are at the peak of this season, there are still many more weeks of activity to come -- and the flu vaccine can help reduce illness and protect against severe complications that can lead to hospitalization or even death. Even during seasons when vaccine viruses and circulating viruses are not well-matched, the flu vaccine may still provide protection."
Vaccines are developed each year based on epidemiological surveillance data collected by the five Collaborating Centers for Reference and Research on Influenza -- the CDC, the Francis Crick Institute in London, the Victoria Infectious Diseases Reference Laboratory in Australia, the National Institute for Infectious Diseases in Japan and the National Institute for Viral Disease Control and Prevention in China. The group identifies the three or four strains of influenza they believe are most likely to spread and cause illness among people during the upcoming flu season.
Flu viruses are constantly mutating, so vaccine composition is reviewed and updated annually, based on the influenza viruses making people sick, the extent to which those viruses are spreading, and how well the previous season's vaccine protects against those viruses.
Viruses may also mutate during flu season, which can render even the latest vaccine ineffective.
"Most flu vaccines are not as effective as we'd like, especially in older adults, who are at greatest risk for more severe disease," Stephen S. Morse, a professor of epidemiology at Columbia University's Mailman School of Public Health, told UPI. "We still have a lot to learn, but I'm hopeful that we can do better. Given the unpredictability of flu, and the risk of pandemics, we have to."
Historically, flu vaccines were made in eggs -- hence, warnings for those with egg allergies in the past -- and antigens, which are substances that stimulate immune response. Vaccines for each of the virus strains in circulation had to be grown separately, a process that was time-consuming and required multiple eggs for each shot.
Over the past decade or so, Morse said, cell-culture vaccines -- in which the antigens are grown in cell cultures, or cells grown in lab environments, instead of eggs -- have become available, as have recombinant vaccines, which use genes of viral antigens for production.
Enhanced production techniques have also, in general, improved vaccine quality and raised hopes that there may one day be a "universal" vaccine: a shot that would be effective against most, if not all, flu virus strains in circulation.
Moody noted that a number of adjuvants -- or substances added to the vaccine formulation to enhance immune response -- are being tested that should improve further safety and effectiveness.
"The idea is that we want to make vaccines that have greater cross-protection against strains circulating in humans, those to which humans can be susceptible, like avian strains, and also to make vaccines that provide more durable protection so that people wouldn't have to get re-vaccinated every year," said Moody, who is working with colleagues on U.S. National Institutes of Health-funded efforts to develop improved vaccines.
"Even if vaccines currently in development do solve the problem, we would be several years away from having a licensed product, mainly because there are multiple steps in testing the vaccines for safety and effectiveness," he added. "That being said, any improvement over the current influenza vaccines is still a 'win,' so we remain open to the idea of making any improvements."