People vaccinated against the COVID-19 are just as likely to spread the delta variant of the virus to their household contacts as those who haven't been inoculated, according to new research reported in NDTV.

In a yearlong study carried on 621 people with mild COVID-19 in the United Kingdom, the scientists observed that their peak viral load was same regardless of vaccination status, as per a paper published on Thursday in The Lancet Infectious Diseases medical journal.

The study also found that 25% of inoculated household contacts still contracted the infection from an index case, while 38% of those who weren't vaccinated became infected.

The findings somewhat explains why the delta variant is so infectious even in countries with good number of vaccinations, and why the unvaccinated people can't assume they are safe because others have been inoculated. Those who were vaccinated returned negative more quickly and only had milder disease, while unvaccinated household members were more likely to suffer from severe infection and hospitalization.

"Our findings show that vaccination alone is not enough to prevent people from being infected with the delta variant and spreading it in household settings," said Ajit Lalvani, a professor of infectious diseases at Imperial College London who co-led the study. "The ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves."

It was found that vaccination reduces household transmission of the alpha variant -- first reported in the United Kingdom in late 2020 -- by between 40% and 50%, and infected inoculated people had a lower viral load in the upper respiratory tract than those who hadn't taken vaccine. However, the delta variant has been the dominant strain around the world for some time.

The study also showed that immunity acquired after full vaccination waned in as little as three months. The authors of the research said there wasn't enough data to advise on whether this should lead to an amendment in the UK's booster policy, where third doses are currently being administered to older and more prone individuals six months after their second shot.

The period of six months was chosen arbitrarily following early data from Israel on the efficacy of boosters, but there is no reason to believe they would be less efficient if administered earlier, said Neil Ferguson, an epidemiologist at Imperial College London and investigator on the study, at a press briefing Thursday.

The booster shots could help curb the virus, as extra shots or repeated infections tend to lead to longer immunological memory, potentially protecting individuals for up to a year, Mr Lalvani said. More data is required to confirm this, he added.

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