Breakthrough infection transmissions common, study finds

The Covid-19 Delta variant can spread easily from vaccinated people to their household contacts, even if they’re fully vaccinated, according to new research from the United Kingdom.

Researchers found that while full vaccination reduced the risk of infection with the Delta variant and accelerated viral clearance, vaccinated people with breakthrough infections had similar peak viral loads to unvaccinated cases.

The analysis showed a higher secondary household transmission rate among vaccinated people than had been reported prior to the emergence of the Delta variant.

In the cohort study of densely sampled household contacts exposed to the Delta variant, published online Oct. 28 in the journal Lancet Infectious Diseases, 25% of vaccinated household contacts tested positive for Covid-19 compared to 38% of unvaccinated household contacts.

“Our findings show that vaccination alone is not enough to prevent people from being infected with the Delta variant and spreading it in household settings,” researcher Ajit Lavani, FMedSci, of Imperial College London, noted in a press statement. “The ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe Covid-19, especially as more people will be spending time inside in close proximity during the winter months.”

The study also found that vaccine protection decreased within a few months of receiving a second Covid-19 vaccine dose, thus increasing risk of infection, which makes a strong case for booster dosing, Lavani noted.

Lavani and colleagues identified 602 community contacts of 471 Covid-19 index cases between September 2020 and September 2021 using the U.K.’s extensive contact tracing system.

The researchers collected demographic and vaccination information at enrollment. Household and non-household exposed contacts agreed to daily self-swabbing of the upper respiratory tract for up to 20 days, and during this period the contacts contributed 8,145 respiratory tract samples.

Transmission risk by vaccination status was analyzed for 231 contacts exposed to 162 epidemiologically linked Delta variant-infected index cases.

The researchers compared viral load trajectories from fully vaccinated participants with Delta infection (n=29) with unvaccinated participants with Delta (n=16), Alpha (B.1.1.7; n=39), and pre-Alpha (n=49) infections.

The primary outcome for the epidemiological analysis was assessment of the probability of infection among susceptible people after exposure to an infectious household contact, know as the secondary attack rate (SAR).

In their viral load kinetics analysis, the researchers sought 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.

Among the main study findings:

  • SAR in household contacts exposed to the Delta variant was 25% (95% CI, 18–33) among fully vaccinated participants compared with 38% (95% CI, 24–53) among unvaccinated people.
  • 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 versus 23% [15–31] for unvaccinated).
  • 39% of infections in fully vaccinated household contacts resulted from fully vaccinated epidemiologically linked index cases, “further confirmed by genomic and virological analysis in three index case–contact pairs.”
  • Peak viral load did not differ by vaccination status or variant type, but it did “increase modestly” with age (difference of 0.39 [95% credible interval −0.03 to 0.79] in peak log10 viral load per mL between those ages 10 years and 50 years).
  • Fully vaccinated people 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.

“Our findings help to explain how and why the Delta variant is being transmitted so effectively in populations with high vaccine coverage,” Lavani and colleagues wrote. “Although current vaccines remain effective at preventing severe disease and deaths from Covid-19, our findings suggest that vaccination alone is not sufficient to prevent all transmission of the Delta variant in the household setting, where exposure is close and prolonged.”

In accompanying commentary, professor of infectious diseases Annelies Wilder-Smith, MD, PhD, of London School of Hygiene & Tropical Medicine, noted that even though vaccination against Covid-19 continues to provide strong protection against hospitalization and death, the impact of the Delta variant on transmission is concerning.

“This study confirms that Covid-19 vaccination reduces the risk of Delta variant infection and also accelerates viral clearance in the context of the Delta variant,” Wilder-Smith wrote. “However, this study unfortunately also highlights that the vaccine effect on reducing transmission is minimal in the context of Delta variant circulation.”

Wilder-Smith wrote that the finding has immediate implications for public health.

“Higher vaccination coverage rates need to be achieved because indirect protection from vaccinated to unvaccinated people remains sub-optimal,” Wilder-Smith wrote. “The question of whether booster doses will improve the impact on transmission should be addressed as a top priority. Research efforts should be directed towards enhancing existing vaccines or developing new vaccines that also protect against asymptomatic infections and onward transmission. Until we have such vaccines, public health and social measures will still need to be tailored towards mitigating community and household transmission in order to keep the pandemic at bay.”

  1. The U.K. study showed that Covid-19 Delta variant can spread easily from vaccinated people to their household contacts, including fully vaccinated contacts.

  2. Researchers found that while full vaccination reduced the risk of infection with the Delta variant and accelerated viral clearance, vaccinated people with breakthrough infections had similar peak viral loads to unvaccinated cases.

Salynn Boyles, Contributing Writer, BreakingMED™

Funding for this research was provided by the National Institute for Health Research, and others.

Researcher Neil Ferguson reported consulting fees from the World Bank; payment or honoraria from the Wellcome Trust; travel expenses from WHO; advisory board participation for Takeda; and is a senior editor of the eLife journal. All other authors and commentary writer Wilder-Smith declares no competing interests.

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Topic ID: 79,190,730,933,190,926,192,927,151,928,925,934