A Danish household survey shows that COVID vaccines reduce susceptibility and transmission

Severe acute coronavirus 2 respiratory syndrome (SARS-CoV-2) has undergone recurrent and extensive mutational changes that have led to several variants of concern (VOCs), which often show higher transmission and immune avoidance characteristics. Among these VOCs includes the Delta variant, which has played a key role in the latest wave of infections, hospitalizations and deaths worldwide.

Study: Effect of vaccination on the transmission of SARS-CoV-2 Delta VOC at home.  Image Credit: TY Lim / Shutterstock.com

study: Effect of vaccination on SARS-CoV-2 Delta VOC transmission in the household. Image Credit: TY Lim / Shutterstock.com


Researchers are looking to understand the real effectiveness of the vaccine (VE) in terms of the effectiveness of the vaccine for sensitivity (VES), the effectiveness of the vaccine for transmission (VET) and the combined effect (VEC).

Some of the challenges associated with determining VE in the general population include earlier vaccination of high-risk groups with a high vaccination rate, which may lead unvaccinated people to show the effects of bias in the study. As humoral immunity declines over time, this adds to the difficulty in identifying the true effectiveness of vaccines.

An earlier study showed that vaccinated individuals tended to transmit about half of the unvaccinated levels. The present study uses national data from Denmark to help validate this finding.

Survey results

The researchers found that VES against the SARS-CoV-2 Delta variant was 61% of the unvaccinated primary case, but 46% in the vaccinated. VET is also observed in 31% and 10% in unvaccinated and vaccinated individuals at risk of infection, respectively.

The VEC was 66% if both the primary case and the secondary contacts were fully vaccinated. These findings confirm previous results.

In general, VET is higher among the unvaccinated and the addition of a third booster dose reduces protection against transmission compared to the double vaccinated. VET did not show much difference from the SARS-CoV-2 Omicron variant compared to the Delta strain.

Vaccination is known to prevent infection and hospitalization and reduce the number of severe and fatal infections due to the Delta variant. The overall conclusion from the results of the present study shows that vaccination also prevents the transmission of SARS-CoV-2 from vaccinated individuals.

That is, Delta VOC causes more sudden infections in vaccinated people, perhaps due to the avoidance of immunity, combined with a higher rate of replication in the airways. However, vaccination has been shown to be associated with a reduced viral load in sudden infections, which may be one of the ways in which vaccination reduces the tolerability of the Delta variant.

VES decreased from 71% one month after vaccination to 32% every eight months. The combined VET also showed a downward trend from 57% to 29% at the same time. These observations support the current booster dose recommendation to reduce transmission and improve immune protection against the Delta variant.

Ct values ​​in vaccinated and unvaccinated positive secondary cases.


The results of the study show that vaccination reduces the transmission of SARS-CoV-2 and should therefore be used to reduce the risk of infection among susceptible populations that come into contact with infected individuals. Current COVID-19 vaccines are more effective in reducing the susceptibility of vaccine recipients to the virus, despite the fact that they do not achieve sterilizing immunity.

For this reason, the use of N95 masks should be encouraged in order to reduce transmission from vaccinated individuals with outbreaks to unvaccinated and susceptible individuals. Conversely, vaccinated individuals are less susceptible to SARS-CoV-2, which may help fully vaccinated individuals to mix more freely in public places.

This again supports the issuance of immunity passports, although this is a politically contentious issue.

A third consequence is reduced transmission from vaccinated individuals who have had a breakthrough infection. To this end, vaccination is important to protect people or workers in society, including nursing homes, and should therefore be a priority to help control the pandemic.

Notes on tests and cases: This figure shows the number of tests performed per million Danish citizens and the number of new positive cases detected per thousand Danish citizens from June to November 2021. Data source: SSI (2021a).

Notes on tests and cases: This figure shows the number of tests performed per million Danish citizens and the number of new positive cases detected per thousand Danish citizens from June to November 2021. Data source: SSI (2021a).

Children aged 10 years or younger have been found to be involved in the transmission of viruses, especially Delta VOC, with a secondary attack (SAR) of 27%. Up to 20 years of age, the SAR is 15% for unvaccinated children in relation to vaccinated household members aged two to 60 years.

Evaluations of this study could be very useful in modeling the course of the pandemic, as they provide measures of sensitivity and portability. The main strength of this study is that the unvaccinated group consists mainly of those who do not yet qualify for vaccination, rather than those who are unwilling or unable to receive the vaccine. This has avoided a major source of potential bias, as earlier studies have shown that unvaccinated individuals are less likely to follow other public health recommendations.

Ultimately, VE assessments here only reflect a lower level of protection. Household members are likely to have the same vaccination status due to similar age, ease of transportation to and from the vaccination site, and beliefs surrounding vaccination.

Vaccines are generally expected to be more protective at the Community level than households due to more intense exposure in households, which increases the likelihood of sudden infection. In the latter, the virus variant is likely to be adapted to the antibodies induced by the vaccine, which increases its tolerability.

The researchers concluded that vaccination reduced both susceptibility to SARS-CoV-2 and transmission among unvaccinated primary and potential secondary cases by 61% and 32%, respectively, with respect to the Delta variant.

*Important message

medRxiv publishes preliminary scientific reports that are not reviewed by partners and therefore should not be considered convincing, guiding clinical practice / health-related behavior, or treated as established information.

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