Fully vaccinated people can contract and pass on COVID-19 in the home, but at lower rates than unvaccinated people.

These are the findings of a study of COVID-19 transmission between household contacts, led by Imperial College London and the UK Health Security Agency (HSA) and published today in The Lancet Infectious Diseases.

It finds that people who have received two doses of vaccine have a lower, but still appreciable, risk of becoming infected with the delta variant in the home compared with people who are unvaccinated. The authors stress that vaccination also reduces the risk of severe illness, hospitalisation and death from COVID-19.

The ongoing transmission we are seeing... makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe COVID-19. Professor Ajit Lalvani NHLI

The analysis found that around 25% of vaccinated household contacts tested positive for COVID-19 compared with roughly 38% of unvaccinated household contacts.

Fully vaccinated people cleared the infection more quickly than those who are unvaccinated, but their peak viral load – the greatest amount of SARS-CoV-2 virus found in their nose and throat – was similar to that seen in unvaccinated people, which may explain why they can still readily pass on the virus in household settings.

According to the researchers, the study is one of few to date conducted using detailed data from households and offers crucial insights into how vaccinated people can still be infected with the delta variant and pass it to others.

Despite transmission between vaccinated people being possible, the researchers say it is essential for people who are unvaccinated, and those who are now eligible for boosters, to get vaccinated against COVID-19 to protect themselves from severe disease and hospitalisation.

“Vaccines are critical to controlling the pandemic, as we know they are fantastic at preventing serious illness and death from COVID-19,” said Professor Ajit Lalvani, of the National Heart & Lung Institute at Imperial College London, who co-led the study.

“Our findings show that vaccination alone is not enough to prevent people from being infected with the delta variant, and from spreading it onwards, in household settings. This is likely to be the case for other indoor settings where people spend extended periods of time in close proximity as will occur increasingly as we head into winter.”

Household COVID-19 spread

In the study, carried out by the NIHR Health Protection Research Unit in Respiratory Infections at Imperial, researchers enrolled 621 participants, identified by the UK contact tracing system, between September 2020 and September 2021 – before vaccine boosters had become widely available in the UK.

All participants had mild COVID-19 illness or were asymptomatic (showing no symptoms), and took swabs from their nose and throat each day for 14-20 days. 

Of the 621 participants, 163 tested positive for COVID-19. Whole genome sequencing confirmed that 71 were infected with the delta variant of the SARS-CoV-2 virus, 42 had alpha and 50 had the original strain of SARS-CoV-2. Of the 71 participants infected with delta, 23 (32%) were unvaccinated, 10 (14%) received one vaccine dose and 38 (54%) had received two vaccine doses.

Continued public health and social measures to curb transmission thus remain important, even in vaccinated individuals. Dr Anika Singanayagam Department of Infectious Disease

A total of 205 household contacts of delta variant index cases were identified, of which 53 tested positive for COVID-19. Of the 205 contacts, 126 (62%) had received two vaccine doses, 39 (19%) had received one vaccine dose, and 40 (19%) were unvaccinated.

Among household contacts who had received two vaccine doses, 25% (31/126 contacts) became infected with the delta variant compared with 38% (15/40) of unvaccinated household contacts.

Among vaccinated contacts infected with the delta variant, the median length of time since vaccination was 101 days, compared with 64 days for uninfected contacts. This suggests that the risk of infection increased within 3 months of receiving a second vaccine dose, due to waning protective immunity. The authors point to this finding as important evidence of the need for vaccinated people to get a booster shot as soon as they are eligible for one.

Peak viral load

PCR tests on participants’ swab samples were used to detect changes in the amount of virus in a person’s nose and throat (viral load) over time.

A total of 133 participants had their daily viral load trajectories analysed, of whom 49 had pre-alpha and were unvaccinated, 39 had alpha and were unvaccinated, 29 had delta and were fully vaccinated, and 16 had delta and were unvaccinated.

The analysis found that viral load declined most rapidly among vaccinated people infected with the delta variant compared with unvaccinated people with delta, alpha, or pre-alpha.

However, the peak levels of virus in vaccinated people were similar to those in unvaccinated people. The researchers believe this may explain why the delta variant is still able to spread despite vaccination.

Dr Anika Singanayagam, first author of the study and Honorary Clinical Research Fellow within the Department of Infectious Disease at Imperial, said: “Understanding the extent to which vaccinated people can pass on the delta variant to others is a public health priority. By carrying out repeated and frequent sampling from contacts of COVID-19 cases, we found that vaccinated people can contract and pass on infection within households, including to vaccinated household members.

“Our findings provide important insights into the effect of vaccination in the face of new variants, and specifically, why the delta variant is continuing to cause high COVID-19 case numbers around the world, even in countries with high vaccination rates. Continued public health and social measures to curb transmission thus remain important, even in vaccinated individuals”

Professor Ajit Lalvani added: “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. We found that susceptibility to infection increased already within a few months after the second vaccine dose - so those offered a booster should get it promptly.”

The work was carried out through one of several Health Protection Research Unit’s (HPRU) at Imperial College London. Funded by the NIHR, these units are collaborations between Universities, the UK Health Security Agency (UK-HSA) and other partner organisations and aim to provide centres of excellence in multi-disciplinary health protection research.

The research was carried out in response to the surges of alpha and then the delta infections seen in the UK. The researchers say the work is an exemplar of a research intensive university partnering with the national public health agency to deliver important policy-relevant findings in a short time frame. It was made possible thanks to the existing structure of the NIHR HPRU in Respiratory Infections, renewed in April 2020.

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‘Community transmission and viral load kinetics of SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: an observational, longitudinal, cohort study’ by Anika Singanayagam et al. is published in The Lancet Infectious Diseases.

This article is based on materials from The Lancet.

Frequently asked questions

Does the study show that vaccinated and unvaccinated become infected and pass on Covid-19 at the same rate?

No. Regarding ‘becoming infected’: vaccinated persons are less likely to become infected than unvaccinated persons but they are still at risk of acquiring infection in the household setting (which involves close and prolonged exposure) with 25% of vaccinated contacts becoming infected. 

Our study also estimated that around 38% of unvaccinated household contacts become infected. However, the sample sizes were small and the confidence intervals correspondingly wide, i.e. precision was low. The infection rate (secondary attack rate) among vaccinated contacts could therefore be as low as 18% and that among unvaccinated contacts could be as high as 53%, which is consistent with vaccination conferring substantial protection against infection. 

Furthermore, since the study was a snapshot of current transmission in real-life UK households where most unvaccinated persons are teenagers and children, the unvaccinated contacts were on average younger than the vaccinated contacts. 

Since younger people have a somewhat lower risk of acquiring infection than older adults, an ideal head-to-head comparison of infection rates in vaccinated vs unvaccinated contacts would need to compare large groups of the same age range, which was not possible given that most UK adults are vaccinated, and most UK teenagers not yet vaccinated. This limitation was acknowledged in the ‘Discussion’ section of the article. 

The overall conclusion is therefore that vaccinated contacts have appreciable risk of acquiring infection from household members with COVID-19 but this risk is lower than in unvaccinated contacts.

Regarding ‘passing on infection’: the study found that vaccinated COVID-19 cases (breakthrough infections) infected 25% of their household contacts, indicating that even vaccinated cases are infectious to their close contacts. 

For contacts exposed to unvaccinated cases, a similar proportion (23%) became infected. However, the sample sizes were small and the confidence intervals correspondingly wide, i.e. precision was low. The transmission from vaccinated cases could therefore be as low as 15% and that from unvaccinated cases as high as 31% which is consistent with vaccinated cases being half as infectious as unvaccinated cases. 

Moreover, the study was a snapshot of current transmission in real-life UK households where most unvaccinated persons are teenagers and children. Since these younger age groups are known to be somewhat less infectious than older adults, an ideal head-to-head comparison of the infectiousness of vaccinated vs unvaccinated cases would need to compare large groups of the same age range, which was not possible given that most UK adults are vaccinated and most UK teenagers not yet vaccinated. 

The overall conclusion is therefore that vaccinated breakthrough cases can efficiently transmit infection but most likely at a lower rate than age-matched unvaccinated persons.

Some use the study as evidence of the redundancy of the vaccines and the vaccine passport. Is this correct?

No. Vaccines are highly effective and are having a decisive impact on the pandemic both for individuals and whole populations. There is ample published evidence that definitively shows that vaccines are effective at preventing SARS-CoV-2 infection (including with Delta variant) and highly effective at preventing severe illness, hospitalisation and death from COVID-19. There is no sense in which COVID-19 vaccines can be considered redundant. 

The implication of our results for vaccine passport mandates is that if one attends an event where everyone has a vaccine passport, it does not guarantee that no transmission will occur between those present - but the transmission would be much less than at an event where no-one is vaccinated. 

Some experts have noted that in the study's analysis on the SAR, no adjustment or stratification for age is done. Therefore, as the median age for unvaccinated is lower than for the vaccinated, the study is not informative of the comparison between vaccinated and unvaccinated in respect of the rate of passing on the virus. Is this true?

The study provided a snapshot of current transmission in real-life UK households where most unvaccinated persons are teenagers and children. Compared to older adults, these younger age groups are known to be less likely to become infected and somewhat less infectious when they are infected. 

The overall conclusion of the study is therefore that vaccinated contacts can become infected in households and vaccinated breakthrough cases can efficiently transmit infection to household members - but very probably at a lower rate than would be seen in age-matched unvaccinated persons. 

An ideal comparison of vaccinated vs unvaccinated cases would need to compare large groups of the same age range, but this was not possible because most UK adults are vaccinated and most UK teenagers are not yet vaccinated. 

This was mentioned among other limitations of the study in the penultimate paragraph of the ‘Discussion’ section of the article and is also addressed above in detail.

Professor Ajit Lalvani, Chair in Infectious Diseases, National Heart and Lung Institute, Imperial College London, and co-lead on the ATACCC study.

Dr Anika Singanayagam, Honorary Clinical Research Fellow, Department of Infectious Disease, Imperial College London.