Herd immunity a distant dream, say experts

Achieving herd immunity is like “a castle in the sky that’s impossible to reach,” said government adviser Yuen Kwok-yung, adding the feat is no longer attainable due to the emergence of the Delta variant.

Yuen and his team estimate the target can only be met when vaccine coverage reaches over 97.4 percent amid the global spread of the highly contagious variant.

But people should still get jabbed as early as possible as the shots can lower the risk of developing serious symptoms and death, even from the Delta variant, Yuen – along with fellow University of Hong Kong researchers Christopher Lung and Kelvin Chiu Hei-yeung – wrote in a Chinese-language newspaper.

The three said the city can only reopen borders and relax social distancing measures after all residents fit for vaccination have completed both doses. But the city cannot survive if it does not resume economic activities with the mainland and the international community.

Local authorities are pushing to maintain “zero Covid,” but experts say looking at results overseas, it is unlikely the SAR can stay free of outbreaks forever.

“Zero infection should not serve the purpose of eradicating Covid-19, but to buy time for citywide vaccination,” they wrote. “People should seize the current zero infection status and get jabbed as early as possible.”

They said vaccines from various brands, such as BioNTech/Fosun’s Comirnaty, offer as much as 95 percent protection against the original Covid-19 strain. In these cases herd immunity can be reached upon a 70 percent coverage. But with the far more contagious Delta variant, the vaccine’s protection drops to around 88 percent and an uptake rate of 97.4 percent is needed for herd immunity.

The other vaccine option in the SAR is Sinovac’s CoronaVac with 60 percent efficacy, which requires a coverage of 142.9 percent to achieve herd immunity, the researchers said.

“An inoculation rate of over 100 percent may seem illogical, but it shows that the requirement for community immunity has been hugely lifted due to non-stop mutations in Covid-19 strains, which speed up transmission and lower vaccine protection,” the experts wrote.

“So herd immunity is like a castle in the sky that’s impossible to reach.”

The experts made four suggestions to minimize the risks from the Delta variant. In terms of border policies, vaccinated arrivals who also test positive for antibodies can enjoy a shortened quarantine on the condition of undergoing virus tests every other day.

People must show vaccination proof to attend mask-off or large-scale events. If the SAR sees outbreaks of more infectious mutant variants, schools must switch to a hybrid of in-person classes for immunized students and online ones for the unvaccinated. All indoor premises must improve their ventilation to avoid the emergence of clusters.

The experts said that even if the SAR’s inoculation coverage reaches 90 percent, people should continue wearing masks outside.

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