Double vaccinated three times less likely to test positive for COVID-19 than unvaccinated, says study

By Angela Betsaida B. Laguipo, BSNAug 6 2021

Vaccination efforts against coronavirus disease 2019 (COVID-19) have commenced in many parts of the world. To date, over 4.31 billion doses have already been administered globally.

While mass vaccination has had a tangible impact on the containment of COVID-19, particularly where uptake has been high, the emergence of variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has raised concerns about the overall efficacy of current vaccines. This is because many of the current models are presently based on the wild-type virus, which was first detected in Wuhan, China, in December 2019.

Some variants of concern have exhibited increased infectivity and even some evidence of partial resistance to neutralizing antibodies elicited from earlier strains and the currently employed vaccines. Of particular concern has been the Delta variant, which first emerged in India earlier this year.

In new research, scientists at Imperial College London report that double vaccinated individuals could be three times less likely than unvaccinated people to test positive for SARS-CoV-2 infection.

Study: REACT-1 round 13 final reports: exponential growth, high prevalence of SARS-CoV-2, and vaccine effectiveness associated with Delta variant in England during May to July 2021. Image Credit: LookerStudio / Shutterstock

The study, which appeared as a pre-print report on Imperial’s Spiral server, analyzed results of polymerase chain reaction (PCR).

The team’s findings also suggest that fully vaccinated individuals may be less likely to pass the virus on to others since they have lesser viral loads than unvaccinated individuals.

The REACT-1 Study

The study results came from the Imperial-led REACT-1 study, a significant coronavirus monitoring program. The researchers analyzed prevalence trends and their drivers using reverse transcription-polymerase chain reaction (RT-PCR) swab-positivity data from round 12, between May 20 and June 7, 2020, and round 13, between June 24 and July 12, 2021, of the Real-time Assessment of Community Transmission-1 (REACT-1) study, with the team sending swabs to non-overlapping random samples of people aged 5 and above in England.

During the study period, 0.63 percent of people were infected. This represents about a four-fold increase compared to the study’s last report, when 0.15 percent had the virus as of June 7.

Vaccinated people and SARS-CoV-2 infection

The study studied swab tests of nearly 100,000 people in England. They performed swab testing at home, and their samples were analyzed by PCR testing. Of these, 537 people tested positive, with an overall prevalence of 0.63 percent. Further, about 254 of those who tested positive were infected with the Delta variant.

Those who were unvaccinated had a three-fold higher prevalence of infection than those who had received both doses of a vaccine, at 1.21 percent compared to 0.40 percent. However, in round 13, 44 percent of infections happened in fully vaccinated people, reflecting imperfect efficacy against infection despite high overall vaccination levels.

The team estimated vaccine effectiveness against infection in round 13 of 49 percent, among participants between 18 and 64, which increased to 58 percent when considering only strong positives. Also, the team estimated the adjusted vaccine effectiveness against symptomatic infection at 59 percent.

The team concluded that between May and July 2021, despite highly successful vaccination efforts with high vaccine uptake, infections were driven by the Delta variant and high infection prevalence among younger and unvaccinated individuals. Despite many people receiving two doses to reduce transmission effectively, the fast spread of the Delta variant makes controlling the viral spread difficult.

“Although slower growth or declining prevalence may be observed during the summer in the northern hemisphere, increased mixing during the autumn in the presence of the Delta variant may lead to renewed growth, even at high levels of vaccination,” the researchers concluded in the study.

The team added that without additional interventions to mitigate the pandemic’s impact, increased mixing during the autumn in the presence of the Delta variant may cause a renewed growth of cases, even at high levels of vaccination. They suggested that continued surveillance to monitor the viral spread is needed globally.