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High COVID-19 vaccination rates were expected to reduce transmission of SARS-CoV-2 in populations by reducing the number of possible sources for transmission and thereby reducing the burden of COVID-19 disease.

Recent data, however, indicate that the epidemiological relevance of COVID-19 vaccinated individuals is increasing. In the UK it was described that secondary attack rates among household contacts exposed to fully vaccinated index cases were similar to household contacts exposed to unvaccinated index cases (25 per cent for vaccinated vs 23 per cent for unvaccinated).

12 of 31 infections in fully vaccinated household contacts (39 per cent) arose from fully vaccinated epidemiologically linked index cases. Peak viral load did not differ by vaccination status or variant type []. In Germany, the rate of symptomatic COVID-19 cases among the fully vaccinated (“breakthrough infections”) is reported weekly since 21. July 2021 and was 16.9 per cent at that time among patients of 60 years and older [].

This proportion is increasing week by week and was 58.9 per cent on 27. October 2021 (Figure 1provides clear evidence of the increasing relevance of the fully vaccinated as a possible source of transmission. A similar situation was described for the UK. Between weeks 39 and 42, a total of 100,160 COVID-19 cases were reported among citizens of 60 years or older. 89,821 occurred among the fully vaccinated (89.7 per cent), 3395 among the unvaccinated (3.4 per cent) [].

One week before, the COVID-19 case rate per 100.000 was higher among the subgroup of the vaccinated compared to the subgroup of the unvaccinated in all age groups of 30 years or more. In Israel, a nosocomial outbreak was reported involving 16 healthcare workers, 23 exposed patients and two family members. The source was a fully vaccinated COVID-19 patient. The vaccination rate was 96.2 per cent among all exposed individuals (151 healthcare workers and 97 patients).

Fourteen fully vaccinated patients became severely ill or died, the two unvaccinated patients developed mild disease []. The US Centres for Disease Control and Prevention (CDC) identifies four of the top five counties with the highest percentage of the fully vaccinated population (99.9–84.3 per cent) as “high” transmission counties [].

Many decision-makers assume that the vaccinated can be excluded as a source of transmission. It appears to be grossly negligent to ignore the vaccinated population as a possible and relevant source of transmission when deciding about public health control measures.

Figure 1. Vaccination rates and proportions of fully vaccinated people among symptomatic COVID-19 cases (≥ 60 years) in Germany between 21. July and 27. October 2021 based on the weekly reports from the Robert Koch Institute [].


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    • Singanayagam A, Hakki S, Dunning J, et al.
    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.

    Lancet Infect Dis. 2021;

  2. Robert Koch-Institut. Wöchentlicher Lagebericht des RKI zur Coronavirus-Krankheit-2019 (COVID-19). AKTUALISIERTER STAND FÜR DEUTSCHLAND 22. Juli 2021. (accessed 28. September 2021).
  3. UK Health Security Agency. COVID-19 vaccine surveillance report. Week 4328. Oktober 2021. (accessed 1. November 2021).
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    The nosocomial outbreak caused by the SARS-CoV-2 Delta variant in a highly vaccinated population, Israel, July 2021.

    Euro Surveill. 2021; 262100822

    • Subramanian SV, Kumar A.
    Increases in COVID-19 are unrelated to levels of vaccination across 68 countries and 2947 counties in the United States.

    Eur J Epidemiol. 2021;

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