The immune responses against RSV remained above prevaccination levels for at least 18 months after the second vaccination in the parent study of an adjuvanted vaccine against RSV.

Now, a new study reported that a third vaccine dose was well tolerated and recalled the immune responses in older adults (OAs).

“Together with the ongoing confirmatory clinical trials, these results help better characterize this RSV vaccine, in terms of safety and RSV-specific immune responses elicited in older adults,” wrote the authors of a study in The Journal of Infectious Diseases.

The researchers from Ghent University and Ghent University Hospital in Belgium and colleagues evaluated safety and immune responses after revaccination with a third dose of an adjuvanted vaccine against RSV in OAs aged 60 to 80 years. “Revaccination was done with the licensed vaccine formulation, which was also selected for further investigation in several phase III clinical trials,” the researchers explained.

Background information in the article noted that RSV is a common, contagious seasonal virus causing respiratory tract infections. Recently, several vaccines against RSV in OAs have been licensed in the U.S. and the European Union.

“In older adults, RSV can cause serious respiratory illnesses or worsen underlying medical conditions such as chronic diseases of the lungs or heart failure,” the researchers point outed. “Severe disease may lead to hospitalization, increased need for oxygen, and ventilatory support.”

The article added that the two main antigenic subtypes, RSV-A and RSV-B, circulate with alternating predominance across seasons, with varying patterns.

Usually, especially in immune-competent older children and younger adults, RSV infections resolve without complications or sequelae. In adults aged 60 years and older, however, RSV can cause more serious respiratory illnesses, including lower respiratory tract disease.

That is especially the case in patients with underlying medical conditions or those who are immunocompromised, according to the authors, who added that the significant disease burden has been historically underestimated.

“The severity of RSV-associated disease in OAs has been ascribed to waning humoral and cellular immune responses (immunosenescence) that were induced by previous RSV infections,” the authors of the current study wrote. “A protective immune response against RSV is orchestrated by antibodies (e.g., immunoglobulin A [IgA] and IgG, neutralizing antibodies [nAb]), and lymphocytes (both cluster-of-differentiation-4-expressing [CD4+] and CD8+ T cells) that produce a variety of cytokines such as interleukins (ILs) and interferons (IFNs), resulting in viral clearance and protection. The RSV-specific immunity obtained after infection is not long-lasting and, even though most people have some level of post-infection immunity, this does not prevent subsequent RSV infections. Due to a higher disease burden in the vulnerable OA population, the waning immune responses lead to an increased risk for more severe disease in OAs. Thus, approaches to overcome waning immunity (e.g., vaccination) can help avoid serious RSV-associated disease in OAs.”

The RSV vaccine investigated in this study is based on prefusion stabilized in its trimeric conformation (RSVPreF3) as the main antigen, and adjuvanted with AS01E.

“With the world population aging, disease prevention and reduced disease burden are important focus points of public health care. RSV is a common pathogen that can lead to severe respiratory disease in the OA population,” the researchers pointed out. “OAs are susceptible to developing infection-associated morbidities and may be unable to mount an effective protective response against RSV. An RSV vaccine tailored toward the OA population will thus need to maximize the elicited immune responses, to overcome age-related immunosenescence, and to protect OAs against RSV-associated disease.”

The researchers noted that their extension study “provides further insights into vaccine-induced immune responses in the OA population.”

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