COVID vaccines protect against Delta but don't fully stop disease spread

Mom on couch with sick child
Mom on couch with sick child

Katarzyna Bialasiewicz / iStock

While COVID-19 vaccines lower the likelihood of infection with the highly transmissible SARS-CoV-2 Delta (B1617.2) variant, the virus can still be transmitted within households—but less so than among unvaccinated people—a study today in The Lancet Infectious Diseases finds.

Led by Imperial College London researchers, the study evaluated community COVID-19 transmission and viral loads among 602 vaccinated and unvaccinated contacts of 471 mildly ill coronavirus outpatients ages 5 years and older from Sep 13, 2020, to Sep 15, 2021. For up to 20 days, 133 participants contributed 8,145 upper respiratory tract samples, regardless of whether they had symptoms.

Of 205 household contacts of COVID-19 patients infected with Delta, 62% had received two vaccine doses, 19% had received one dose, and 19% were unvaccinated.

Higher infection rates among unvaccinated

The secondary attack rate, or the percentage of contacts infected by the index patient, was 25% (95% confidence interval [CI], 18% to 33%) in fully vaccinated participants, compared with 38% (95% CI, 24% to 53%) in the unvaccinated.

Among household contacts of fully vaccinated COVID-19 patients, the secondary attack rate was comparable to that of household contacts of unvaccinated index patients (25% [95% CI, 15% to 35%] for vaccinated vs 23% [95% CI, 15% to 31%] for unvaccinated patients).

Of 31 infections, 12 (39%) in fully vaccinated COVID-19 household contacts arose from epidemiologically connected fully vaccinated index patients; genomic and virologic analysis in three pairs of index cases and contacts found a similar percentage.

While vaccinated participants cleared the coronavirus faster than their unvaccinated counterparts, both had similar peak viral loads, which the researchers said could explain its ease in spreading among household members, regardless of vaccination status. Nor did peak viral load differ by variant type, but it increased some with age (difference in peak log10 viral load per milliliter [mL] between those aged 10 and 50 years, 0.39 [95% credible interval, -0.03 to 0.79]).

Average viral load also declined faster in fully vaccinated participants infected with the Delta variant than among unvaccinated participants infected with pre-Alpha (B117), Alpha, or Delta variants.

Median time between receipt of a second vaccine dose and study recruitment in fully vaccinated contacts was longer for infected versus uninfected participants (101 vs 64 days), which the study authors said could be attributed to waning vaccine-induced immunity.

Mitigation steps still needed post-vaccine

Co-senior author Ajit Lalvani, MD, of Imperial College London, said in a Lancet news release that the findings demonstrate that vaccine alone is not enough to prevent infection with and transmission of the Delta variant.

"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," he said. "We found that susceptibility to infection increased already within a few months after the second vaccine dose—so those eligible for COVID-19 booster shots should get them promptly."

In a commentary in the same journal, Annelies Wilder-Smith, MD, PhD, of the London School of Hygiene & Tropical Medicine, said the study underscores the importance of vaccination, because vaccinated contacts were better protected against Delta infection than the unvaccinated.

"All breakthrough infections were mild, and no hospitalisations and deaths were observed," she wrote. "But these results also highlight that breakthrough infections continue to occur in the vaccinated."

Wilder-Smith noted that the study showed that vaccine-related reduction of Delta variant transmission among household contacts is minimal and said the findings have urgent public health implications in terms of achieving higher COVID-19 vaccination rates and studying whether booster doses will lower viral transmission.

"Research efforts should be directed towards enhancing existing vaccines or developing new vaccines that also protect against asymptomatic infections and onward transmission," she concluded. "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."

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