Friday, October 29, 2021

The Lancet: Community Transmission & Viral Load Kinetics of the SARS-CoV-2 Delta (B.1.617.2) Variant in Vaccinated vs Unvaccinated

Credit CDC 


The Lancet has published a study on viral load and community transmission of the Delta variant in vaccinated vs. unvaccinated individuals, that confirms what we first saw in in July (see The CDC `The War Has Changed' Internal Document On Delta Variant);  that `breakthrough' infections in vaccinated individuals could produce comparable viral loads to unvaccinated individuals. 

Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 2021 - August 6th MMWR

What is added by this report?

In July 2021, following multiple large public events in a Barnstable County, Massachusetts, town, 469 COVID-19 cases were identified among Massachusetts residents who had traveled to the town during July 3–17; 346 (74%) occurred in fully vaccinated persons. Testing identified the Delta variant in 90% of specimens from 133 patients. Cycle threshold values were similar among specimens from patients who were fully vaccinated and those who were not. Weekly / August 6, 2021 / 70(31);1059-1062

This does not - as many on social media seem to think - mean that vaccinated individuals have the same risk of transmitting COVID to close contacts as the unvaccinated. Those who are vaccinated are still substantially less likely to become infected which make them less able to spread the virus. 

But, COVID vaccination provides no guarantee of not becoming infected, or of being unable to spread the virus.  But it does lower the risks. 

First the link, and abstract from today's study, followed by a press release/summary.  Then I'll return with more. 

Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study

Anika Singanayagam, PhD *Seran Hakki, PhD *,Jake Dunning, PhD *Kieran J Madon, MSc

Michael A Crone, MBBChAleksandra Koycheva, BSc, et al.

Open Access Published:October 29, 2021




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.


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]).


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.

         (Continue . . .)

Next, some excerpts from a press release by The Lancet.

The Lancet Infectious Diseases: CoVID-19 vaccines lower risk of infection with delta variant, but infection can still be passed on in household settings
Peer-Reviewed Publication

Peer-reviewed / Observational study / People
  • Study of 621 people in the UK with mild COVID-19 infections found that people who received two vaccine doses could still pass the infection on to vaccinated and unvaccinated household members.
  • The analysis found that 25% of vaccinated household contacts tested positive for COVID-19 compared with 38% of unvaccinated household contacts. The infectiousness of vaccinated cases with breakthrough infections was similar to unvaccinated cases.
  • Infections in vaccinated people cleared more quickly than those in unvaccinated people, but resulted in a similar peak viral load – when people are most infectious – probably explaining why the delta variant remains able to spread despite vaccination.
  • The authors urge unvaccinated people to get vaccinated to protect themselves from severe disease and those eligible for a booster to receive it as soon as offered. They also call for continued public health and social measures to curb transmission, even in vaccinated people.
People who have received two vaccine doses against COVID-19 have a lower, but still appreciable, risk of becoming infected with the delta variant compared with unvaccinated people. Vaccinated people clear the infection more quickly, but the peak viral load among vaccinated people is similar to that seen in unvaccinated people, which may explain why they can still readily pass on the virus in household settings, according to a study published in The Lancet Infectious Diseases.

Vaccines remain highly effective at preventing severe disease and deaths from COVID-19, but some studies suggest they may be less effective against the delta variant – currently the dominant strain worldwide – though the reason for this has not been established. Most COVID-19 transmission is known to occur in households yet there is limited data on the risk of transmission of the delta variant from vaccinated people with asymptomatic or mild infections in the community.

          (Continue . . . )

Almost exactly a year ago - six weeks before the first COVID vaccines were made available to the public, I wrote in A COVID Vaccine Reality Checkthat a successful vaccine would be an important tool in the fight against COVID, but would not be a panacea for this pandemic (excerpt follows)

At this point - even assuming a COVID-19 vaccine can be produced - no one can predict how effective it will be in preventing infection, or lowering the severity of illness, in any age group or demographic. Nor do we know how long any protection from the vaccine will last. 

Admittedly, even a vaccine that is only 50% effective in preventing infection, or even one that substantially reduces the odds of developing severe disease, would be an important tool in fighting this pandemic. 

But most people expect more from a vaccine, particularly given how much hype this one has received.  A modest success could easily appear as a massive failure to most Americans, who are being primed to expect a `magic bullet' against COVID-19. 

Advocates always tend to `oversell' the benefits of a new vaccine, while detractors ignore the good they do, and focus only on the negatives.  The truth usually falls somewhere in between, but almost always favors getting the vaccine. 

We'd almost certainly be in a much bigger mess today without the COVID vaccine, and I am personally grateful to have received two doses, and expect to get the booster in the weeks ahead. 

But vaccines alone are unlikely to break the back of this pandemic.  The coronavirus is too nimble, and current vaccines can only offer so much protection.  Which is why I never stopped wearing my face mask in public after I was vaccinated, and will continue to do so this winter. 

As much as we'd like to declare victory, and return to a `normal' life, the virus doesn't appear anywhere near giving up.  

And unless we are prepared to take a serious hit, neither should we.