Background

CC-BY-NC-ND 4.0 International license  . High  rates of  virus  transmission and the  presence  of  variants of  concern  can affect  vaccine effectiveness (VE). Both  conditions  occur  in  low-income  countries, which  primarily  use  viral vector  or  inactivated virus vaccine  technologies.  However,  few VE analyses have  been conducted  in  such  countries,  and  most  lack  the  power  to  evaluate  effectiveness  in  subgroups, such  as  the elderly.

Methods

The present  retrospective cohort  study  evaluated  the  effectiveness  of  Vaxzevria and CoronaVac  vaccines  for  COVID-19-related  infection  in  60,577,870  Brazilian vaccinees from January  18  to June  30,  2021.  Study  outcomes  included  documented  infection with  severe  acute  respiratory  syndrome coronavirus  2  (SARS-CoV-2), Covid-19–related  hospitalization, ICU  admission  and  death.  We estimated  VE  for  each  outcome  as  one  minus  the  hazard  ratio  using  Cox  regression adjusted  for  age,  sex,  Brazilian deprivation  index,  and month/region  of  dose  administration.

Results

Vaccination  with  Vaxzevria  or  CoronaVac  was  found  to  be  effective  against  SARS-CoV-2 infection  and highly  effective  against  hospitalization,  ICU admission and  death  in  individuals up  to  79  years.  From  80-89  years of  age,  91.2  (95CI:  89.1-92.9)  VE  against  death was seen  in Vaxzevria-vaccinated  individuals  versus  67.3  (95CI:  63.6-70.6)  for  Coronvac. Above  90 years,  70.5  (95CI:  51.4-82.1)  protection  was  conferred  to  Vaxzevria-vaccinated  individuals versus  35.4  (95CI:  23.8-45.1)  in  Coronavac-vaccinated  individuals 

Conclusions

Both  vaccines  demonstrated  overall  effectiveness  against  severe  COVID-19  up  to  80  years  of age.  Our  results  suggest  that  individuals  aged  90  years or  older  may  benefit from  an expedited  third  booster  dose.  Ongoing  evaluations, including  any  additional  vaccines authorized,  are  crucial  to  monitoring  long-term vaccine  effectiveness.

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