With the coronavirus largely stopped at China’s borders it can be hard to assess just how well the national vaccination drive is working against the highly transmissible Delta variant of the disease.

But two small peer-reviewed studies indicate that the vaccines might just be effective.

In one study, researchers with the Chinese Academy of Sciences collected 28 serum samples from participants three weeks after they received the full three doses of a vaccine by Anhui Zhifei Longcom Biopharmaceutical.

They tested the serum to see how well the vaccine, known as ZF2001, neutralised several variants of the coronavirus, according to a study published in the journal The Lancet on Friday.

The researchers, who included Gao Fu, head of the Chinese Centre for Disease Control and Prevention, found that the serum from people given the vaccine showed roughly equivalent sensitivity against the Epsilon variant, the one first detected in the United States, and the Delta variant, first detected in India, as compared with the ancestral strain first detected in Wuhan.

“ZF2001 preserved the neutralising activity against the newly emerging Delta variant,” the researchers wrote.

But it was much less effective against the Beta variant, first identified in South Africa, and the Gamma variant, first identified in Brazil.

According to the paper, the levels of neutralising antibodies dropped “1.1 times” for the Epsilon variant, 1.2 times for the Delta variant, 1.8 times for the Beta variant first identified in South Africa and 1.5 times for the Gamma variant, first identified in Brazil.

Of the 28 people sampled, 16 had their shots over a period of 4-6 months and 12 had the doses over two months.

The results showed that participants with longer intervals between the second and third dose had greater resilience to variants.

“The better performance of the extended interval regimen is probably because of the longer antibody maturation in the recipients than in those with the shorter interval regimen,” the researchers wrote.

“Our data are consistent with common practice of using the 0, 1, and 6 months regimen for subunit vaccines against diseases such as hepatitis B, and provide guidance to further optimise the vaccination regimen.”

ZF2001 was authorised for emergency use in China and Uzbekistan in March this year.

The Uzbekistan government announced on the weekend that the vaccine had been approved for local production.

Researchers in the southern Chinese city of Guangzhou also concluded that two other more widely used inactivated vaccines by Sinovac and Sinopharm were effective in preventing cases caused by the Delta variant.

The assessment, published earlier this month in Emerging Microbes & Infections, was based on observation of a community outbreak in the city in late May and early June when an imported Delta case infected 153 people.

Of the infected, 38 were vaccinated and 115 were not.

The researchers tracked 74 people infected with Covid-19 and 292 close contacts as the controls to explore vaccine efficacy in real-world settings.

The participants were aged 18-59 years and most were vaccinated with the Sinovac jabs. Some people have been given both vaccines, but researchers said they did not separate them in the study because it was not designed to compare the two and both vaccines were based on the same widely available technology.

The team included researchers from the Guangzhou Centre for Disease Control and Prevention, the Guangzhou Institute of Respiratory Health, and Sun Yat-sen University, including respiratory expert Zhong Nanshan.

They found that a single dose of inactivated vaccine yielded a vaccine efficacy of only 13.8 per cent, not sufficiently protective against the Delta strain infection. After two doses, the efficacy rose to 59 per cent against the coronavirus in general and 70.2 per cent against moderate disease.

The efficacy against severe disease was estimated as 100 per cent, but researchers said it might be overestimated because there were only two severe Covid-19 patients in the control group.

There were no severe and critical cases or deaths among the vaccinated study participants, and all the 16 severe or critical cases were not vaccinated.

“We speculated that the inactivated vaccine could prevent severe Covid-19 well,” they wrote. “The two-dose dosing scheme of the inactivated vaccines was effective against the Delta variant infection in real-world settings, with the estimated efficacy exceeding the World Health Organization minimal threshold of 50 per cent.”

They said their findings could have important global implications because more than 350 million inactivated vaccines had been exported worldwide as of June.

But they cautioned that the efficacy results should be considered in the context of stringent containment efforts in China.

The selection of controls, people who live, work or travel with confirmed cases “might have affected the estimated vaccine efficacy”, the researchers wrote.

The Delta variant has put countries around the world on the guard with some considering booster shots for better protection.

A report by the US Centre for Disease Control and Prevention published last week showed two doses of mRNA vaccines were 74.7 per cent effective against infection among nursing home residents in March and May. But this declined significantly to 53.1 per cent during June and July when the Delta variant circulation predominated.

The report suggested an additional dose of Covid-19 vaccine might be considered for nursing home and long-term care facility residents to optimise a protective immune response.

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