**3. RSV strains**

There are two RSV strains as RSV A and RSV B and are categorized on basis of genetic and antigenic differences [10]. However, mostly extensive antigenic and nucleotide sequence variation was observed between RSV A and RSV B, however, genetic as well as antigenic variability was also studied within the individual groups of RSV [11]. Multiple studies demonstrated the differences in viral replication between these two groups; specifically, RSV A replicated to higher titers than RSV B viruses in both cell culture and animal models [12–16]. In addition, RSV A infection is more virulent and severe than RSV B [17].
