COVID-19 is not the only infectious disease that concerns health experts worldwide. Others like influenza, measles, dengue, and HIV can be as deadly as the pandemic ravages international supply lines. Recently, a study highlighted that early infections of influenza A predict the efficacy of flu vaccines. This means people who encountered the disease early on may benefit more from the vaccines.
The association between early infections of influenza A and the efficacy of flu vaccines was investigated by researchers at the University of Chicago and Marshfield Clinic Research Institute. Their findings revealed that early infections could play a role in the protectiveness of the vaccines. People who had been previously exposed to the virus might get more from the vaccines, regardless of the efficacy level. They published their findings in the journal eLife.
Influenza Disease Burden and Related Outcomes of 2018-2019 Season
Unlike COVID-19, the flu is a viral infection caused by various influenza strains. Throughout the year, people can contract influenza but there are specific months when strains are highly abundant in the environment. Oftentimes, cases of influenza infections rise during colder months because the human respiratory system is less robust, compared during warmer months. And in this pandemic, people should not ignore the threat of influenza because the disease can lead to serious complications.
According to the US Centers for Disease Control and Prevention (CDC), a federal agency, the estimated medical visits in the 2018-2019 season was 2,434,180 visits in children aged four years and younger, 3,984,921 visits in children aged five to 17 years, 4,407,885 visits in adults aged 18 to 49 years, 3,972,356 visits in adults aged 50 to 64 years, and 1,721,007 visits in adults aged 65 and older. In terms of hospitalizations, the estimated rates were 25,328 in children aged four years and younger, 21,012 in children aged five to 17 years, 66,869 in adults aged 18 to 49 years, 97,967 in adults aged 50 to 64 years, and 279,384 in adults aged 65 and older. Similar to COVID-19, older adults would likely be hospitalized due to influenza.
Experts at John Hopkins Medicine, a teaching hospital of Johns Hopkins University, said that both diseases attack the respiratory system and can cause similar symptoms, which can confuse even doctors. Either of the two can induce fever, body aches, and pneumonia, however, there can be differentiated from one another. Though, the expertise of a doctor will matter in distinguishing which disease is troubling a patient.
The first distinguishing factor is the pathogen. Only the SARS-CoV-2 virus can cause COVID-19 while several strains of influenza can induce the flu. Another differentiating factor is the transmission risk, wherein COVID-19 is now being considered airborne as respiratory droplets expelled by a sick person can be very small, allowing persistence in the air. On the other hand, influenza's main transmission route is via respiratory droplets. The only scenario that allows influenza to become airborne is when aerosolization procedures are involved.
Early Exposure to Influenza Matters in Vaccine Efficacy
A recent study suggested the correlation of early influenza A infections to vaccine efficacy. Researchers found statistical evidence that early exposure could increase the default efficacy of flu vaccines. The less exposure one has, the lower the efficacy would be. But multiple exposures to the virus would reflect better vaccine benefits. Still, the study proposed nothing about intentional exposure to the disease.
"Since the risk of influenza infection in a given age group changes over time, factors other than age may affect our susceptibility to infection. We wanted to see whether these differences can be explained in part by the protection gained from childhood flu infection, which has lasting impacts on the immune response to future infections and the protection against new influenza A subtypes," explained Philip Arevalo, first author of the study and postdoctoral researcher at the University of Chicago, quoted eLife.
Based on the interim estimates of the CDC dated February 2020, the efficacy of the 2019-2020 seasonal influenza vaccine varied against strains. Out of 4,112 children and adults enrolled in the US Influenza Vaccine Effectiveness Network, from October 23, 2019, to January 25, 2020, the overall efficacy against any influenza virus related to medically attended acute respiratory illness (ARI) was 45%. The efficacy against the influenza B/Victoria viruses has been estimated to be 50%, but against influenza A(H1N1)pdm09, the estimated efficacy was only 37%. Despite the estimates, CDC said that the vaccine formula could provide 55% protection among children, adolescents, and adults.
In the study, researchers utilized statistical models to measure the influence of early exposure to vaccine efficacy. The models were used in flu cases identified by seasonal studies of vaccine effectiveness from 2007-2008 to 2017-2018 flu seasons. The cases were a part of the Marshfield Epidemiologic Study Area (MESA) in Wisconsin. Every flu season, people from a certain community group were recruited in MESA and tested for influenza if they sought outpatient care due to ARI. Eligible individuals, who were older than six months of age, received routine care from the Marshfield Clinic throughout the study period.
Results from the statistical models highlighted the obvious disparity between people who were exposed to influenza A and people who were not. Even though both subtypes H1N1 and H3N2 evolved throughout the study period, the infection rates and requests for medical attention decreased in certain parameters. They found that those who got sick before from either subtype were less likely to be infected again or seek medical attention. But the association was determined higher in H1N1 infection than in H3N2 infection. The vaccine efficacy was found as well to be different in the birth year and age of patients. Those who contracted either subtype early in life received higher protection levels from flu vaccines.
Although researchers detected a connection between vaccine efficacy and influenza A early exposure, the connection could only be established at the epidemiological level. Further studies would be needed to discover the physiological mechanisms behind the connection. If that could be determined, it would help researchers understand the variation in flu vaccine efficacy every year.