Covid-19 vaccines protect against severe forms of the disease but are not as effective in preventing infection and transmission, especially since the Delta variant became more dominant.
In homes where barriers and masks fall, SARS-CoV-2 spreads most easily. Vaccination is an effective means of reducing the risk of infection, but it is far from infallible, especially against the Delta variant. A British study conducted at Imperial College London concluded that a full vaccination program offers only very partial protection – but still more protection than no vaccination – against the Delta variant in the context of internal transmission.
“Our results show that vaccination alone is not enough to prevent people from contracting and spreading the Delta variant outside the home. This is likely to apply to other enclosed spaces where people spend long periods of time in close proximity. As winter approaches, this will happen more and more often,” says Ajit Lalvani, a professor at the National Heart and Lung Institute at Imperial College London, who was involved in the study.
Between September 2020 and September 2021, researchers followed 621 participants who were identified using a contact-tracking system set up in the UK. The study participants, who were generally young and in good health, reported only mild or asymptomatic forms of Covid-19. Of these, 163 were positive for SARS-CoV-2: 71 for the Delta variant, 42 for the Alpha variant, and 50 for the original variant. Of the 71 people infected with the Delta variant, 23 were unvaccinated, 10 were partially vaccinated and 54 were fully vaccinated. These 71 people are index cases, i.e. they are considered to be the starting point of the infection chain. The researchers regularly monitored and tested people with whom the index cases were in contact in their households; these are the contact cases. In total, 71 of the index cases had 205 contact cases, of which 53 were infected with the coronavirus.
Of the 205 contact cases, 126 were fully vaccinated, 39 were partially vaccinated and 40 were unvaccinated. The researchers then compared the vaccination status of contact cases that tested positive for the Delta variant: 25% were fully vaccinated and 38% were unvaccinated. Although vaccination reduces the risk of contracting the Delta variant, the likelihood of infection remains high. Based on these data, researchers at Imperial College London estimated that vaccines are 34% effective in preventing Delta virus infection.
In addition to tracking infections at home, the researchers also monitored viral loads in 133 patients, both vaccinated and unvaccinated. Each day, the amount of coronavirus in their bodies was estimated using PCR. The researchers found that the viral peak, when the number of virions is highest, was the same in both vaccinated and unvaccinated patients. The British researchers believe that this intense peak is what makes the Delta variant so easily transmitted, even in vaccinated people. In contrast, the viral load drops faster in vaccinated people.
The Imperial College London researchers’ paper highlights an important fact: vaccination alone, especially for the Delta variant and at the start of the cold season, is not enough to stop SARS-CoV-2 transmission. Ventilation of enclosed spaces, social distancing, masks, and hand washing are all good practices that break the chains of transmission. In addition, vaccines are still very effective in their main function: preventing severe illness and hospitalizations.