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.

Findings
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]).

Interpretation
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.

Funding
National Institute for Health Research.

https://www.thelancet.com/journals/laninf/article/PIIS1473-3099
(21)00648-4/fulltext

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