懂你英语Level6 Unit3 Part2 Listening Infectious diseases2
Now,acenturylater,wehavefluvaccinesthathavereducedtheseverityoftheepidemics.Thesevaccinesneedtobeupdatedeveryyearbecausetheviruschangesandnewstrainsemerge.Whatresearchersfearisthatanewstrainofthevirus