A teenager receives a dose of a vaccine against the coronavirus disease (COVID-19) as Israel urged more 12- to 15-year-olds to be vaccinated, citing new outbreaks attributed to the more infectious Delta variant, at a Clalit healthcare maintenance organisation in Tel Aviv, Israel June 21, 2021. REUTERS/Amir Cohen

The Pfizer–BioNTech and Oxford–AstraZeneca vaccines have been found at least initially to effectively protect against the fast-spreading Delta coronavirus variant, but the level of immunity is waning over time, according to a study led by the University of Oxford.

The results supported similar observations seen in Israel, which began administering third Pfizer doses last month to counteract a surge in infections of the Delta variant.

Researchers at UK’s University of Oxford conducted in partnership with the country’s Office for National Statistics concluded that two vaccine doses are still the most effective way to ensure protection against the COVID-19 Delta variant, but that their effectiveness is reduced after a few months. The Delta variant has caused sharp rises in infections in many countries, including some with relatively high vaccination coverage such as the UK.

“Whilst vaccinations reduce the chance of getting COVID-19, they do not eliminate it. More importantly, our data shows the potential for vaccinated individuals to still pass COVID-19 onto others, and the importance of testing and self-isolation to reduce transmission risk,” said Koen Pouwels, senior researcher in Oxford University’s Nuffield Department of Population Health.

The study researched the effectiveness of vaccines in a large community-based survey of randomly selected households across the UK, where PCR tests were performed irrespective of symptoms, vaccination and prior infection. One result showed that Delta infections after two vaccine doses had similar peak levels of virus to those in unvaccinated people; in comparison, peak virus levels in those infected post-vaccination with the Alpha COVID-19 variant were much lower.

Another key finding suggested that two doses of the Pfizer-BioNTech vaccine have greater initial effectiveness against new COVID-19 infections, but that it declined faster compared with two doses of the Oxford-AstraZeneca jab. However, over a time period of four to five months, effectiveness of both vaccines was found to be similar. Vaccine effectiveness was also generally higher at ages between 18 and 34 years old.

The Pfizer-BioNTech vaccine was found to be 92% effective in preventing people from developing a high viral load in their tests 14 days after the second dose. But the vaccine’s effectiveness fell to 90%, 85% and 78% after 30, 60 and 90 days, respectively. The Oxford-AstraZeneca vaccine was 69% effective against a high viral load 14 days after full vaccination, falling to 61% by 90 days.

Sarah Walker, Professor of Medical Statistics and Epidemiology at the University of Oxford and chief investigator for the COVID-19 infection survey, said: “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.

“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. This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide,” Walker added.

An Israeli study released Wednesday showed that a third dose of the Pfizer vaccine was found to be 86% in people aged over 60. Some 1.1 million Israelis have already received a third jab, and on Thursday, a government panel recommended that booster shots be offered to all Israelis over the age of 40.