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, lead 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.
