COVID-19 Vaccines Lower Risk of Infection With Delta Variant – But Infection Can Still Be Passed On

A study of 621 people in the UK with mild COVID-19 infections found that people who received two doses of vaccine could still pass the infection on to vaccinated and unvaccinated family members. The analysis found that 25% of vaccinated family contacts tested positive for COVID-19, compared to 38% of unvaccinated family contacts. The infectivity of vaccinated cases with breakthrough infections was comparable to unvaccinated cases. Infections in vaccinated people cleared up faster than those in unvaccinated people, but resulted in a similar peak viral load – when people are most contagious – which probably explains why the delta variant can still spread despite vaccination. The authors urge unvaccinated people to get vaccinated to protect themselves against serious illness, and those who qualify for a booster to get it as soon as possible. They also call for continued public health and social measures to curb transmission, even in vaccinated people.

People who have received two doses of vaccine against COVID-19 have a lower, but still significant, risk of becoming infected with the delta variant compared to unvaccinated people. Vaccinated people cure the infection faster, but the maximum viral load among vaccinated people is similar to that in unvaccinated people, which may explain why they can still easily pass the virus on in domestic settings, according to a study published in The Lancet Infectious Diseases .

Vaccines remain highly effective at preventing serious illness and death from COVID-19, but some research suggests they may be less effective against the delta variant – currently the dominant strain worldwide – although the reason for this has not been established. It is known that most transmission of COVID-19 occurs in households, but there is limited data on the risk of transmission of the delta variant from vaccinated people with asymptomatic or mild infections in the community.

Professor Ajit Lalvani of Imperial College London, UK, who co-led the study, said: “Vaccines are critical to managing the pandemic as we know they are highly effective in preventing serious illness and death from COVID-19. 19. However, our findings show that vaccination alone is not sufficient to prevent humans from becoming infected with the delta variant and spreading it in domestic settings. The continued transmission we see between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from infection and severe COVID-19, especially as more people will spend time indoors during the winter months. Spending time. We found that susceptibility to infection increased as early as a few months after the second vaccine dose – so those who qualify for COVID-19 booster injections should receive it immediately.

The new study enrolled 621 participants between September 2020 and September 2021, identified by the UK’s contact tracing system. All participants had mild COVID-19 illness or were asymptomatic. Demographic and vaccination status information was collected at enrollment and participants were given daily PCR tests to detect infection whether they had symptoms or not. It is one of the few studies conducted to date using such detailed data from real households, and provides important insights into how vaccinated people can still be infected with the delta variant and pass it on to others.

In this study, participants were defined as unvaccinated if they had not received a single dose of COVID-19 vaccine for at least seven days prior to enrollment, partially vaccinated if they had received one dose more than seven days prior to enrollment, and fully vaccinated if they had received two doses. more than seven days in advance. The study took place before vaccine boosters were widely available.

The risk of transmission based on vaccination status was analyzed for contacts with households exposed to delta variant index cases (the first case detected in a household). By performing PCR testing on swabs 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 using PCR data. modeling, allowing comparisons between fully vaccinated cases of delta infection and unvaccinated cases of delta, alpha and pre-alpha infection.

A total of 205 household contacts of delta variant index cases were identified, 53 of whom tested positive for COVID-19. Of the 205 contacts, 126 (62%) received two vaccine doses, 39 (19%) one vaccine dose, and 40 (19%) were unvaccinated. Of the household contacts who had received two vaccine doses, 25% (31/126 contacts) became infected with the delta variant compared to 38% (15/40) of the non-vaccinated household contacts.

Among vaccinated contacts infected with the delta variant, the median time since vaccination was 101 days, compared to 64 days for uninfected contacts. This suggests that the risk of infection increased within three months of receiving a second vaccine dose, probably due to declining protective immunity. The authors point to vaccine withdrawal as important evidence for all eligible people to receive booster shots.

A total of 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 decreased more rapidly in vaccinated people infected with the delta variant (0.95 log10 virus copies/ml/day) compared to unvaccinated people with delta (0.79), alpha (0.82) or pre-alpha (0.69). However, the authors note that vaccinated people do not have a lower peak viral load than unvaccinated people, which may explain why the delta variant can still spread despite vaccination, as people are most contagious during the peak viral load phase.

dr. Anika Singanayagam, co-lead author of the study, said: “It is a public health priority to understand the extent to which vaccinated people can transmit the delta variant to others. By conducting repeated and frequent sampling of contacts of COVID- 19 cases, we found that vaccinated people can contract and transmit infections within households, including to vaccinated household members.Our findings provide important insights into the effect of vaccination in the face of novel variants, and in particular why the delta- variant continues to cause high numbers of COVID-19 cases worldwide, even in highly vaccinated countries, so ongoing public health and social measures to curb transmission – such as wearing masks, social distancing and testing – remain important, even among vaccinated individuals .”

The authors acknowledge some limitations of their study. Due to the nature of symptom-based community testing in the UK, only symptomatic index case contacts were recruited. Since this field study was conducted when the infection was widespread, it cannot be ruled out that another family member may have already been infected and transmitted COVID-19 to the index case (resulting in a misclassification of who is the index case and who is the contact, but this would not affect the conclusion that transmission occurs between vaccinated individuals). Because older age groups were vaccinated earlier during the introduction of the UK vaccine, the age of unvaccinated people infected with the delta variant was lower than that of vaccinated participants, meaning age may be a confounding factor. However, the higher transmission in unvaccinated contacts was likely due to the absence of vaccination rather than a younger age, as susceptibility to COVID-19 infection does not increase with age. Although the authors did not perform viral culture — a better proxy for infectivity — two other studies using the technique reached conclusions consistent with their findings.

Reference: “Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal cohort study” by Anika Singanayagam, PhD; Seran Hakki, PhD; Jake Dunning, PhD; Kieran J Madon, MSc; Michael A Crone, MBBCh; Aleksandra Koycheva, BSc; Nieves Derqui-Fernandez, MSc; Jack L Barnett, MPhys; Michael G Whitfield, PhD; Robert Varro, MSc; Andre Charlett, PhD; Rhia Kundu, PhD; Joe Fenn, PhD; Jessica Cutajar, BSc; Valerie Quinn, BSc; Emily Conibear, MSc; Prof Wendy Barclay, PhD; Prof Paul S Freemont, PhD; Prof. dr. Graham P. Taylor, DSc; Shazaad Ahmad, FRCPath; Prof Maria Zambon, FRCPath and Prof Neil M Ferguson, DPhil, October 29, 2021, The Lancet Infectious Diseases.
DOI: 10.1016/S1473-3099 (21) 00648-4

This study was funded by the National Institute for Health Research. It was conducted by researchers from Imperial College London, University of Oxford, University of Surrey, Public Health England and Manchester Academic Health Sciences Centre, UK.