New Delhi: There is a lower, but still appreciable, risk of getting infected with the Delta variant of SARS-CoV-2 for people who have received both the doses of Covid-19 vaccine, compared with those unvaccinated. 


A new study published in The Lancet Infectious Diseases Journal says the Delta variant can infect vaccinated people in household settings because their peak viral load is similar to that in unvaccinated people. 


The Delta variant is currently the dominant strain of SARS-CoV-2. Vaccinated people can clear the infection quicker than unvaccinated individuals. Vaccines mostly protect people from severe diseases and deaths from Covid-19, but may be less effective against the Delta variant, according to different studies. Infections spread in household settings in vaccinated people are mostly asymptomatic or mild.


Vaccination Alone Not Enough To Prevent Covid-19 Infection


Professor Ajit Lalvani, the lead author of the study, said their findings show that vaccination alone is not enough to prevent people from being infected with the Delta variant and spreading it in household settings. 


He added that it is essential for unvaccinated people to get vaccinated to protect themselves from getting infected with severe Covid-19, because more people will be spending time indoors during winter. 


People eligible for Covid-19 booster shots should get the shots promptly, Lalvani said, adding that the susceptibility to infection after the second dose has been found to increase within a few months of getting the second shot.


Spread Of Infection With Delta Variant In Household Settings


The researchers studied 621 participants, and identified them by the UK contact tracing system, between September 2020 and September 2021. The participants were monitored for a year. They either had mild Covid-19 illness or were asymptomatic. The authors collected the demographic and vaccination status information of the participants when they were enrolled. The participants had to undergo PCR tests on a daily basis, for a certain period of time, to detect infection. This is how the researchers got an idea of how vaccinated people are susceptible to infection with the delta variant. 


Participants who had not received a single Covid-19 vaccine dose at least seven days before enrolment were considered unvaccinated. Those who had received one dose more than seven days before enrolment were considered partially vaccinated. Participants who received both doses more than seven days before being enrolled were classified as fully vaccinated. 


The first case of infection with the Delta variant detected in a particular household is known as a Delta variant index case. Such cases were studied to analyse risk of transmission based on vaccination status. 


The authors, however, noted that contacts of symptomatic index cases only were recruited due to the nature of UK symptoms-based community testing. 


There is a misclassification of who is the index case, and who is the contact. This is because a particular household member may already have been infected and transmitted Covid-19 to the index case.


The participants provided swab samples daily for 14-20 days, on which PCR tests were performed. The amount of virus in a person's nose and throat was estimated by modelling PCR data. This enabled comparisons between fully vaccinated cases of delta infection and cases of delta, alpha, and pre-alpha infection in unvaccinated individuals.


The researchers identified a total of 205 household contacts of Delta variant index cases. Only 53 of them tested positive for Covid-19. The number of people who received two vaccine doses, one vaccine dose, and were unvaccinated was 126, 39, and 40, respectively. As much as 25 per cent of the household contacts who had received two vaccine doses became infected with the Delta variant, compared with 38 per cent of unvaccinated household contacts.


The median length of time since vaccination in uninfected household contacts was 64 days, while that in infected contacts was 101 days, indicating the risk of infection within three months of receiving a second vaccine dose increased due to that waning protective immunity.


The authors note that eligible people must receive booster shots because the effectiveness of the vaccine wanes over time.


Viral Load Decreases More Rapidly In Vaccinated People


The study found that the viral load declined more rapidly among vaccinated people infected with the delta variant, compared with unvaccinated people infected with Delta, Alpha or pre-Alpha variant. 


The authors also note that the peak viral load in vaccinated people was not found to be lower than that in unvaccinated people. People are most infectious during the peak viral load phase. This explains why the Delta variant spreads in people despite vaccination. 


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 pass on the delta variant to others. She explained that continued public health and social measures such as wearing masks, social distancing, and testing, are important even in vaccinated individuals. This is because even vaccinated people can contract and pass on infection to other vaccinated people in their households. 


The findings provide an insight into why there are high Covid-19 cases around the world, even in countries with high vaccination rates.


Citing limitations of the study, a statement issued by The Lancet said the age of unvaccinated people infected with the Delta variant was lower than the age of vaccinated individuals infected with the same strain. This is because the older age groups were vaccinated earlier during the vaccine rollout in the UK. However, younger age susceptibility is not the reason behind higher transmission in unvaccinated household contacts, as Covid-19 infection does not increase with decreasing age, the authors noted.