People who are fully vaccinated against COVID-19 can carry the same viral load after contracting the delta variant than those who are unvaccinated, researchers have found, casting doubt over the potential for reaching herd immunity.

While two doses of Pfizer/BioNTech’s or Oxford/AstraZeneca’s vaccines still offered good protection against the delta variant, the viral load in people infected post vaccination was at similar peak levels than in those unvaccinated, according to a preliminary analysis of UK COVID-19 survey data led by Dr Koen Pouwels, Senior Researcher at the Nuffield Dept of Population Health, University of Oxford.

The implications for viral transmission are still unclear, but the researchers said the finding “requires urgent investigation”, particularly as it indicates that “herd immunity is likely unachievable for emerging variants”.

“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,” said Sarah Walker, professor of medical statistics and epidemiology at the University of Oxford, and Academic Lead the COVID-19 Infection Survey. “But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped.”

Using data from the Office for National Statistics COVID-19 Infection Survey, the researchers looked at swab results from 384,543 adults between December 2020 and May 2021, when the alpha variant was the predominant, and from 358,983 adults between May and August 2021, when delta was the main circulating strain.

They found that, during the delta period, two doses of either BNT162b2 or ChAdOx1 continued to induce a level of protection that was at least on par with that attained via natural infection with the virus in those not vaccinated, with efficacy results of 80%, 67% and 72%, respectively.

Timing between the doses did not seem to have any significant impact on effectiveness, though the researchers did find that having had COVID-19 prior to vaccination increased the level of protection. In people vaccinated with BNT162b2 who had previously had COVID-19, the rate of new infections fell by 93% compared to 85% in those who hadn’t, while for people vaccinated with ChAdOx1 the figures were 88% and 68%, respectively.

Also, while two doses of BNT162b2 had greater initial effectiveness against new COVID-19 infections than ChAdOx1, the level of protection against high viral burden and symptomatic infection declined faster, with results suggesting that after 4 to 5 months effectiveness of the vaccines would be similar, the researchers noted.

Some experts believe that this finding supports the argument for a third booster dose to strengthen protection against the virus.

“Vaccine effectiveness appears to decline more rapidly after completion of a course of BNT162b2 vaccine than following ChAdOx01 vaccine, suggesting that a booster dose may be required to sustain effectiveness and indeed has just been recommended in the US. Although protection does not decline as rapidly following receipt of the ChAdOx01 vaccine, its lower overall protection suggests that a booster dose with a mRNA vaccine might enhance protection against this variant as winter approaches,” said Dr Penny Ward, Faculty of Pharmaceutical Medicine, Visiting Professor in Pharmaceutical Medicine, Kings College London, as quoted by the Science Media Centre.

“On this evidence, it certainly supports the case for third ‘booster’ jabs for vulnerable individuals, as is now happening in Israel,” added Dr Simon Clarke, Associate Professor in Cellular Microbiology at the University of Reading.