Professor Ajit Lalvani of Imperial College London, UK, who co-led the study, said: “Vaccines are critical to controlling the pandemic, as we know they are very effective at preventing serious illness and death from COVID-19.
However, our findings show that vaccination alone is not enough to prevent people from being infected with the delta variant and spreading it in household settings.
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, especially as more people will be spending time inside in close proximity during the winter months.
We found that susceptibility to infection increased already within a few months after the second vaccine doseso those eligible for COVID-19 booster shots should get them promptly.”
This study enrolled 621 participants, identified by the UK contact tracing system, between September 2020 and September 2021. All participants had mild COVID-19 illness or were asymptomatic. Demographic and vaccination status information were collected on enrolment, and participants had daily PCR tests to detect infection, regardless of whether or not they had symptoms.
This study is offering key insights into how vaccinated people can still be infected with the delta variant and pass it on to others.
Participants were defined as unvaccinated if they had not received a single COVID-19 vaccine dose at least seven days before enrolment, partially vaccinated if they received one dose more than seven days before enrolment, and fully vaccinated if they received two doses more than seven days beforehand. The study took place before vaccine boosters had become widely available.
Risk of transmission based on vaccination status was analyzed for household contacts exposed to delta variant index cases.
By performing PCR tests on swab samples provided daily by each participant for 14-20 days, changes over time in viral load – the amount of virus in a person’s nose and throat – were estimated by modelling PCR data, enabling comparisons between fully vaccinated cases of delta infection, and unvaccinated cases of delta, alpha, and pre-alpha infection.
205 household contacts of delta variant index cases were identified, of whom 53 tested positive for COVID-19. Of the 205 contacts, 126 received two vaccine doses, 39 had received one vaccine dose, and 40 were unvaccinated.
Among household contacts who had received two vaccine doses, 25% became infected with the delta variant compared with 38% 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 three months of receiving a second vaccine dose, likely due to waning protective immunity. The authors point to vaccine waning as important evidence for all eligible people to receive booster shots.
133 participants had their daily viral load trajectories analyzed, 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.
Viral load declined more rapidly among vaccinated people infected with the delta variant compared with unvaccinated people with delta, alpha, or pre-alpha .
Vaccinated people did not record a lower peak viral load than unvaccinated people, which may explain why the delta variant can still spread despite vaccination as people are most infectious during the peak viral load phase.
Dr Anika Singanayagam, co-lead author of the study, 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.
The study provide important insights into the effect of vaccination in the face of new variants, and 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 – such as masking wearing, social distancing, and testing – thus remain important, even in vaccinated individuals.”
Limitations: Due to the nature of UK symptoms-based community testing, only contacts of symptomatic index cases were recruited.
As this real-world study was undertaken when infection was circulating widely, it cannot be excluded that another household member may already have been infected and transmitted COVID-19 to the index case.
As older age groups were vaccinated earlier during the UK vaccine rollout, the age of unvaccinated people infected with the delta variant was lower than for vaccinated participants, meaning age could be a confounding factor.
Higher transmission in unvaccinated contacts was due to the absence of vaccination rather than younger age as susceptibility to COVID-19 infection does not increase with decreasing age.
While the authors did not perform viral culture – a better proxy for infectiousness -two other studies that used the technique reached conclusions consistent with their findings.
Source: Medindia
