Main menu


Delta infection: Unvaccinated and vaccinated people have similar levels of virus


The COVID-19 Infection Survey regularly tests a random sample of people in the United Kingdom for SARS-CoV-2.

Researchers have analyzed the study data taken before and after the Delta variant became dominant in the U.K.

The researchers found that vaccinated people with the Delta variant could have a viral load similar to those who had not received a vaccination.

A new study found that people vaccinated against coronavirus who have also contracted the Delta variant of SARS-CoV-2 could have similar peak levels of the virus as people who have not had a vaccination.

The researchers also found that while the vaccination offers significant protection against the Delta variant, the vaccines are less effective than against previous variants.

The researchers drew on data from the COVID-19 Infection Survey, a study led by the Office for National Statistics, the Department for Health and Social Care, and the University of Oxford in the United Kingdom.

Stay informed with live updates on the current COVID-19 outbreak and visit our coronavirus hub for more advice on prevention and treatment.


Vaccines are a key tool in the world’s response to the COVID-19 pandemic. While measures such as washing hands, social distancing, and wearing masks are still important, the effectiveness of the vaccines has been a success story of the pandemic.

However, this effectiveness is reducing as variants of SARS-CoV-2 emerge.

Currently, the Delta variant of SARS-CoV-2 is a central concern. The Delta variant has rapidly spread in the U.K., making up an overwhelming number of cases.

According to Dr. Hans Henri P. Kluge, the World Health Organization’s (WHO) regional director for Europe, “[w]e are far from out of the woods in terms of the pandemic ending, and sadly, in many countries in our region, we are seeing a significant rise in cases associated with the spread of the highly transmissible Delta variant.”

“Despite tremendous efforts by Member States to vaccinate people across the region, millions more remain unvaccinated and therefore at risk of ending up in [the] hospital.”

“The good news is that the data clearly shows that receiving a full vaccination series significantly reduces the risk of severe disease and death. When called to do so, people should get vaccinated,” says Dr. Kluge.

While the vaccines still offer protection against the Delta variant, they are less effective than against previous variants, such as Alpha. Delta is more infectious than previous variants, and researchers, drawing on the latest population data, have found that this is the case for people who have and have not had the vaccination.

This was a finding of the REACT-1 study conducted by Imperial College London, which regularly samples tens of thousands of people. Preprint data from the final round of the study found the vaccines were 49% effective against the Delta variant — a significant drop from previous levels of effectiveness.

Now, in a new preprint, researchers have provided further evidence backing up this finding.

The new study found that virus levels in a fully vaccinated person who has contracted the Delta variant are similar to those of a person who has not had a vaccination.

This is a concern, as a person with a high viral load may be more likely to pass on the virus.

National survey

The new study draws on data from the COVID-19 Infection Survey. This survey tests around 179,000 people every 2 weeks across the U.K. for signs of SARS-CoV-2 using a reverse transcription polymerase chain reaction (PCR) test.

The survey involves testing around 150,000 people every 2 weeks to check antibodies to the virus, using a blood sample.

The study looked at two time periods: December 1 – May 16, 2021, and May 17 – August 1, 2021. The Alpha variant was dominant in the U.K during the first period, whereas Delta was dominant in the second.

Similar peak viral levels

The researchers found that both unvaccinated and vaccinated people who had contracted the SARS-CoV-2 Delta variant had similar peak levels of the virus.

Previously, vaccinated people who contracted the Alpha variant had lower peak levels of the virus than people who had not had a vaccination.

The researchers also found that the Pfizer-BioNTech and Oxford-AstraZeneca vaccines provided a good level of protection against the virus. However, in line with the pre-print results from the REACT-1 study, they were less effective than against the Alpha variant.

The scientists showed that people who had had two doses of the vaccination had at least as much protection against the virus as people who had antibodies after having COVID-19. People who had developed antibodies after having the disease and received both vaccinations had even better protection.

The time between a person having the two doses of either the Pfizer-BioNTech or the Oxford-AstraZeneca vaccines did not make a difference to the level of protection the vaccines offered.

Prof. Sarah Walker of the Nuffield Department of Medicine at the University of Oxford, UK, and the Chief Investigator and Academic Lead for the National COVID-19 Infection Survey and co-author of the present study spoke to the Guardian newspaper. She said that scientists need to conduct more research to determine whether the higher viral load of the Delta variant found in those with and without vaccinations is a cause for concern.

“We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated — for example, they may have high levels of virus for shorter periods of time,” says Prof. Walker.

Delta variant of SARS-CoV-2: Can vaccine boosters stop its spread?

The Delta variant of SARS-CoV-2, the virus that causes COVID-19, is more transmissible than preexisting variants, and it has rapidly become the dominant variant in several countries, including India and the United Kingdom. Some reports suggest that existing COVID-19 vaccines may be less effective in preventing infection with Delta. Can additional booster shots help?

Over the past few months, the Delta variant of SARS-CoV-2 has spread widely in countries around the world, becoming the dominant variant in many places.

Its rapid spread has recently led countries, such as Australia, to reinforce strict lockdowns, as emerging dataTrusted Source suggest the variant is more infectious than preexisting ones, such as the Beta variant, and that it may be able to bypass existing COVID-19 vaccines in some cases.

Prof. Sir Andrew Pollard, head of the Oxford Vaccine Group, which has contributed to the development of the Oxford-AstraZeneca vaccine, has even commented that, in his opinion, the highly transmissible Delta variant has made achieving herd immunity an impossibility.

“The Delta variant will still infect people who have been vaccinated. And that does mean that anyone who’s still unvaccinated at some point will meet the virus […], and we don’t have anything that will [completely] stop that transmission,” he told The Guardian.

Additionally, recent data have also suggested the immunity provided by COVID-19 vaccines fades considerably over time, which also means vaccinated individuals become more susceptible to infection with SARS-CoV-2.

However, some scientists and pharmaceutical companies argue that offering an additional booster shot of some of the most widely authorized COVID-19 vaccines could provide an effective way to keep the Delta variant at bay.

But what does the evidence say so far, and how are countries worldwide responding to the notion of incorporating additional booster shots in their COVID-19 vaccination campaigns?

Preliminary data suggest booster effectiveness

While published data on the effectiveness of COVID-19 vaccine boosters against the Delta variant are not yet available, some of the pharmaceutical companies that produce and distribute COVID-19 vaccines have announced that recent clinical trials support this perspective.

According to Pfizer’s 2021 second-quarter earnings report, receiving an additional booster dose of their COVID-19 vaccine after having had the initial two doses increases the amount of Delta variant antibodies fivefold in 18-to-55-year-olds and 11-fold in 65–85-year-olds.

In answer to queries from Medical News Today, a Pfizer spokesperson explained that this “conclusion is based on initial data from the ongoing booster trial of a third dose of the current BNT162b2 vaccine and laboratory tests.”

“The booster trial builds on the phase 1/2/3 trial and is part of the companies’ clinical development strategy to determine the effectiveness of a third dose against evolving variants,” they noted, adding that Pfizer “expect[s] to publish more definitive data about the analysis in the coming weeks.”

This third dose would be identical to the two doses of the currently authorized Pfizer vaccine. However, the company is also investigating how an “updated” vaccine dose, altered to target the Delta variant specifically, would fare.

The company made this announcement, initially, in its own second-quarter financial report, which states that “[r]obust antibody responses have been observed from existing Moderna booster candidates against COVID-19 in phase 2 studies.”

“In a phase 2 study, vaccination with 50 [micrograms] of three different Moderna mRNA booster candidates induced robust antibody responses […] against important variants of concern, including Gamma (P.1); Beta (B.1.351); and Delta (B.1.617.2),” the report also states.

The three boosters under investigation included their currently authorized shot, as well as two more experimental candidates.

According to Moderna’s report, the levels of neutralizing antibody generated after the third booster shot were similar to those registered after two 100 microgram doses of their currently authorized vaccine.