Administering booster shots to vulnerable demographics such as the elderly can significantly decrease the risk of severe illness and death caused by COVID-19, which makes it a critical part of a country's strategy against the pandemic, experts said on Thursday.

Data show that heterologous immunization, which uses a different type of vaccine than the initial shots for booster shots, can produce a strong immune response that can guard against multiple COVID-19 strains, they added.

Gao Fu, head of the Chinese Center for Disease Control and Prevention, said the biggest challenges surrounding COVID-19 vaccines currently are waning immunity and breakthrough infections by the more transmissible Omicron variant.

Developing more potent vaccines and administering booster shots are some of the countermeasures to address these issues, Gao said during a webinar hosted by the New England Journal of Medicine Frontiers in Medicine on Thursday.

Research data has shown that ZF2001, a protein subunit vaccine developed by the Chinese Academy of Sciences and Anhui Zhifei Longcom Biopharmaceutical, can elicit strong immune protection if given as a booster shot for people who had been inoculated with two doses of inactivated vaccine, Gao said.

This protection, which is measured in the amount of serum neutralizing antibody, is nearly double that of the level achieved with three doses of inactivated vaccine, and this effect applies to numerous mutated strains of COVID-19, he added.

ZF2001 is one of the five vaccines to be granted conditional market approval in China and is the world's first approved protein subunit vaccine, Gao said. As of Oct 29, about 192 million doses of ZF2001 had been administered globally, with a very good safety profile of only 0.21 per 100,000 people reporting abnormal reactions.

The most common side effects include swelling, fever and thickening of the skin at the injection site, he added.

Benjamin Cowling, an epidemiology professor at the University of Hong Kong, said the recent COVID-19 outbreak in Hong Kong was caused by the highly transmissible Omicron BA.2 sub-variant, and the majority of severe and fatal cases had occurred in unvaccinated individuals, especially unvaccinated seniors.

However, if a senior older than 60 was to get booster shots, the protection rate against severe and fatal disease can be around 98 percent for three doses of CoronaVac inactivated vaccine and three doses of Pfizer-BioNTech mRNA vaccine, he said.

"We are confident that a third dose is really providing a high level of protection for older people," he said, adding that it is exciting to see these vaccines perform this well in protecting against severe disease and death caused by the Omicron variant.

Moreover, Cowling's team discovered that just one dose of CoronaVac or the Pfizer-BioNTech vaccine can offer some protection against death in all ages, though the mRNA vaccine performs substantially better in this regard for older demographics.

As a result, it is still valuable to vaccinate seniors during an epidemic, even though they may not finish the full course of inoculation on time, he said. However, Cowling notes that vaccine hesitancy is high among Hong Kong's elderly, especially those with underlying health conditions.

Moreover, given the study was done recently, it is still unclear how long this protection will last, he said.

Fu Yangxin, a professor of medicine at Tsinghua University, said they are designing the next generation of protein fusion vaccine against COVID-19, called V-01, which has the potential to be administered as a nasal drop to enhance mucosal immunity against the pathogen.

The vaccine is currently in phase 3 clinical trials in Pakistan and Malaysia, he said. Scientists are testing its potency as a booster shot for people who received two doses of inactivated vaccines from those countries.

Preliminary data show that the vaccine efficacy rate of V-01 booster is around 61.2 percent for people aged 65 and above, as well as those with underlying health conditions.

An intranasal vaccine may be a game changer in the fight against the pandemic since it can directly boost mucosal defense in a person's upper airway, the place where the Omicron variant mainly targets, Fu said. Since no injection is required, this type of vaccine may be more accepted by children and the elderly, he added.