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COVID-19 Transmissibility and Vaccination

 Can Vaccination Reduce COVID-19 Transmission?

    It's no secret that the B.1.617.2, or delta variant, has been found to be highly transmissible compared to other variants and has thus spread quickly across the globe. Although data prior to the spread of Delta found that vaccination reduces COVID-19 transmission, new reports find that Delta is more likely than other variants to spread among vaccinated individuals. Yes, it is still crucial for those who can to get vaccinated because of the protection it offers. But while many people believe that vaccinated individuals are less likely to transmit the virus, recent studies suggest otherwise. A study published in the medRxiv preprint server examines the viral load and transmission rates of those who are vaccinated and unvaccinated with the COVID-19 vaccine.

About the Study

    The study was conducted in multiple Wisconsin counties including Dane County, a populous county which has one of the highest rates of vaccination in the United States. From June 28 to July 24, 2021, data was collected from 291 individuals who tested positive for COVID-19. Of these individuals, 79 were fully vaccinated and 112 did not receive any vaccine. Sequence data was available for 50 of the 291 individuals. Of these 50 individuals, 42, or 81%, were infected with the Delta variant. In addition to the high prevalence of delta variants in Wisconsin at the time, the data suggests that a majority of unsequenced infections were also caused by delta. The researchers measured the Ct, or cycle threshold values for each of the individuals. Ct values indicate the viral concentration in a sample and the lower the Ct value, the higher the viral load.

Study Results

    Out of the 79 vaccinated individuals with breakthrough infections, 84% had Ct values < 30. Of the unvaccinated individuals, 83% also had Ct values < 30. Meanwhile, 33% of vaccinated individuals with breakthrough infections had extremely low Ct values of < 20 and high viral loads. Researchers found no difference in viral loads when comparing unvaccinated individuals to vaccinated individuals with breakthrough infections. Additionally, individuals with breakthrough infections 'frequently test positive with viral loads consistent with the ability to shed infectious diseases', suggesting that infected vaccinated individuals may be just as likely to transmit COVID-19 as those who are unvaccinated and infected. 

A) Ct values for all COVID-19 positive specimens. B) Ct values for COVID-19 positive specimens confirmed to be infected with the delta variant.

Similar Study

    Collected data following a large gathering in Barnstable County, Massachusetts also had similar findings. Almost 75% of 469 new COVID-19 cases were in vaccinated individuals. Additionally, of the 199 samples sequenced, 90% of individuals were found to have been infected with the Delta variant. Furthermore, researchers found no significant difference in viral load between infected vaccinated and unvaccinated individuals.

What does this mean?

    Although these studies suggest that vaccination does not lower COVID-19 transmission rates with the delta variant, there is still great value in vaccination because of the protection from infection that is provided by the vaccines. Despite the evidence that viral loads of vaccinated and unvaccinated individuals are similar, the immune responses against the virus are very different between the two groups. Those who are vaccinated have a strong immune response to COVID-19 infection that is effective in preventing COVID-19 hospitalization and death. However, with the knowledge that both vaccinated and unvaccinated individuals can experience infection with high viral loads, it's important that everyone, even those who are vaccinated, continues to follow masking and social distancing guidelines to reduce transmission. 

Sources

1. Riemersma, Kasen, et al. “Shedding of Infectious Sars-Cov-2 despite Vaccination.” MedRxiv, 31 July 2021, doi:10.1101/2021.07.31.21261387.

2. Brown, Catherine M., et al. “Outbreak of SARS-CoV-2 Infections, Including Covid-19 Vaccine Breakthrough Infections, Associated with Large Public GATHERINGS — BARNSTABLE County, MASSACHUSETTS, July 2021.” MMWR. Morbidity and Mortality Weekly Report, vol. 70, no. 31, 6 Aug. 2021, pp. 1059–1062., doi:10.15585/mmwr.mm7031e2.

3. Subbaraman, Nidhi. “How Do Vaccinated People Spread DELTA? What the Science Says.” Nature News, Nature Publishing Group, 12 Aug. 2021, www.nature.com/articles/d41586-021-02187-1#ref-CR1.

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