Covid vaccinations ‘not sufficient’ in preventing Delta variant spread, almost equal to unvaccinated – UK study
Vaccination reduces the risk of delta variant infection and accelerates viral clearance. Nonetheless, fully
vaccinated individuals with breakthrough infections have peak viral load similar to unvaccinated cases and can
efficiently transmit infection in household settings, including to fully vaccinated contacts. Host–virus interactions
early in infection may shape the entire viral trajectory.
Background The SARS-CoV-2 delta (B.1.617.2) variant is highly transmissible and spreading globally, including in
populations with high vaccination rates. We aimed to investigate transmission and viral load kinetics in vaccinated
and unvaccinated individuals with mild delta variant infection in the community.
Methods Between Sept 13, 2020, and Sept 15, 2021, 602 community contacts (identified via the UK contract-tracing
system) of 471 UK COVID-19 index cases were recruited to the Assessment of Transmission and Contagiousness of
COVID-19 in Contacts cohort study and contributed 8145 upper respiratory tract samples from daily sampling for up
to 20 days. Household and non-household exposed contacts aged 5 years or older were eligible for recruitment if they
could provide informed consent and agree to self-swabbing of the upper respiratory tract. We analysed transmission
risk by vaccination status for 231 contacts exposed to 162 epidemiologically linked delta variant-infected index cases.
We compared viral load trajectories from fully vaccinated individuals with delta infection (n=29) with unvaccinated
individuals with delta (n=16), alpha (B.1.1.7; n=39), and pre-alpha (n=49) infections. Primary outcomes for the
epidemiological analysis were to assess the secondary attack rate (SAR) in household contacts stratified by contact
vaccination status and the index cases’ vaccination status. Primary outcomes for the viral load kinetics analysis were
to detect differences in the peak viral load, viral growth rate, and viral decline rate between participants according to
SARS-CoV-2 variant and vaccination status.
The SAR in household contacts exposed to the delta variant was 25% (95% CI 18–33) for fully vaccinated
individuals compared with 38% (24–53) in unvaccinated individuals. The median time between second vaccine dose and
study recruitment in fully vaccinated contacts was longer for infected individuals (median 101 days [IQR 74–120]) than
for uninfected individuals (64 days [32–97], p=0·001). SAR among household contacts exposed to fully vaccinated index
cases was similar to household contacts exposed to unvaccinated index cases (25% [95% CI 15–35] for vaccinated vs 23%
[15–31] for unvaccinated). 12 (39%) of 31 infections in fully vaccinated household contacts arose from fully vaccinated
epidemiologically linked index cases, further confirmed by genomic and virological analysis in three index case–contact
pairs. Although peak viral load did not differ by vaccination status or variant type, it increased modestly with age
(difference of 0·39 [95% credible interval –0·03 to 0·79] in peak log10 viral load per mL between those aged 10 years and
50 years). Fully vaccinated individuals with delta variant infection had a faster (posterior probability >0·84) mean rate of
viral load decline (0·95 log10 copies per mL per day) than did unvaccinated individuals with pre-alpha (0·69), alpha (0·82),
or delta (0·79) variant infections. Within individuals, faster viral load growth was correlated with higher peak viral load
(correlation 0·42 [95% credible interval 0·13 to 0·65]) and slower decline (–0·44 [–0·67 to –0·18]).