Delta can make vaccinated people highly infectious, but jabs still work: Experts

A young Hongkonger who returned from the United States caused a stir earlier this month when he not only tested positive for Covid-19, but was also found to be highly infectious.

What made the case unusual was that the 27-year-old man was fully vaccinated in Hong Kong before his two-week trip to the US.

But medical experts insist there is no cause for alarm, with Professor David Hui Shu-cheong of Chinese University saying it was thanks to Hong Kong's tough quarantine and contact-tracing regime that there had been no outbreak so far from the case.

Experts said the man's increased infectiousness was due to the fast-spreading, highly transmissible Delta variant of the coronavirus, and emphasised that vaccines helped mainly to protect against severe illness and death.

On Friday, US media reported an internal document from the Centres for Disease Control and Protection which said the Delta variant left vaccinated and unvaccinated people with "similarly high" viral loads, warning that the variant was as contagious as chickenpox.

Over the weekend, Professor Yuen Kwok Yung, a leading microbiologist and government pandemic adviser, said new information had emerged showing the Delta variant could lead to more vaccinated people spreading the virus under the radar.

He said that presented an even greater risk if people chose not to get tested on the basis they were vaccinated and did not think they could get infected.

Yuen added that some young, fully vaccinated patients in the city were asymptomatic, meaning they could become hidden carriers if their infections went undetected.

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The Hong Kong case of the 27-year-old came to light after the city's Centre for Health Protection took the unusual step of revealing details of what happened after the man returned on July 18 and tested positive.

It said the man had a high viral load, which meant a large amount of the virus was found his upper respiratory tract, making him highly infectious.

His viral load tests showed a "CT value" of about 21 to 24, where any number below 30 indicates a high degree of infectiousness.

He was found with the more transmissible L452R strain, a mutation of the virus also present in the Delta variant.

The centre indicated he was likely to have been infected recently in the US, even though he received two doses of the BioNTech vaccine on April 26 and May 17 in Hong Kong, before he left for the US on July 4. The reason for his trip was not revealed.

The authorities swung into action after he tested positive, imposing an overnight lockdown at the block in Central where the man lived, as it was less than 21 days since he was likely to have been infected. All 185 occupants underwent mandatory testing, but no other infections or carriers were found.

The man's close contacts were quarantined for 21 days, and even their household members were isolated to prevent any possibility of the virus spreading.

According to official data, 30 people in Hong Kong have been found to be infected with Covid-19 despite being vaccinated. On Sunday, two imported cases involved patients who were fully vaccinated, one with the Sinovac jab and the other using BioNTech's.

Chinese University's Hui, who advises the government on the pandemic, said vaccines were most effective at preventing hospitalisation and deaths.

The vaccine creates antibodies, which are proteins in the blood that identify and fight infections, and that protection is spread across the whole body to protect key organs.

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Hui said antibodies could still appear in parts of the body that secreted fluid, such as the nostrils or upper respiratory tract, but in insufficient quantities to tackle and suppress the virus.

That is why a Covid-19 nasal swab test is able to pick up a high viral load.

Hui said a CT value below 30, like the 27-year-old man's, meant it was a live virus and the patient was infectious. But he added that the man might have had an even higher viral load if he was not vaccinated.

He said the quick and thorough response by Hong Kong health authorities helped to ensure that the man did not spread the virus to others, averting a potential outbreak.

Dr Leung Chi Chiu, a respiratory medicine specialist, said the emergence of the Delta variant had changed the understanding of the protection Covid-19 vaccines could offer.

Changes in biological characteristics of the variant increased its transmissibility, causing a higher viral load in those infected, Leung explained.

Such high viral loads have been found in fully vaccinated people in countries with high vaccination rates such as Britain and Israel.

Experts have said a Delta-infected patient can transmit the virus to as many as six to eight others. The variant is expected to be the dominant strain globally.

"There should not be any outbreaks [among the vaccinated groups] if their viral levels were low after getting infected," Leung said.

"Those outbreaks [in countries with high vaccination rates] clearly indicated that the virus had certain transmissibility, which is based on a substantial viral level."

He explained that when an individual was found with a high viral load, the greater concern was over the chances of the virus spreading than the person developing severe symptoms.

It was the body's immune response that determined how severely ill an infected person became, not the amount of the virus in the body, Leung added.

This article was first published in South China Morning Post.