When Covid-19 vaccines were rolled out in China earlier this year, the great hope was that a way out of the pandemic had been found. Once enough people had been vaccinated, enough of a community barrier against the coronavirus would have been built, so that outbreaks could be limited and the country would be free to reopen safely to the world, that was the hope. Now, as much as 76 per cent of China’s population is fully vaccinated, but herd immunity – along with the hoped-for relaxation of pandemic measures and opening of the borders – still remains out of reach. With the virus mutating into ever more contagious and risky variants, vaccination has been expanded to younger age groups and booster doses offered to make up for waning protection more than six months on. Strong public health interventions such as strict quarantine rules, vigorous testing and contact tracing are still being quickly put in place at the first sign of a local outbreak. China has not officially set a vaccination target for building herd immunity, although some government-affiliated experts have said that 80 to 85 per cent of the population would now need to be fully vaccinated to achieve this. “With the highly contagious nature of the Delta variant, I don’t think herd immunity will be possible with the currently available vaccines. Unvaccinated individuals should not rely on herd immunity for protection,” said Dr Jeff Kwong, an epidemiologist and associate professor with the University of Toronto.
“Vaccinated individuals may still get infected, and the goal is for most of them to only have mild infections that won’t lead to any burden on hospital services when they are exposed [to the virus],” he added.Herd immunity against an infectious disease is reached when a sufficient number of people have developed immunity, through vaccination or previous infection, and that threshold for the original strain was estimated to be about 70 per cent. However, new variants such as Delta have changed the equation. The Delta variant, currently the dominant strain in the pandemic, can infect more people than the original strain. Whereas someone infected with the original strain could infect up to three others, for highly transmissible Delta, this number goes up to at least six. This means more people in the susceptible population must be vaccinated, raising the threshold for herd immunity. Ashley St John, associate professor at the Duke-NUS Medical School, said that a stronger effect of the inoculation programme on slowing the spread of the disease could be expected, as high vaccination levels work to limit new infections. However, in reality, a high enough vaccination rate for herd immunity would be hard to attain. “It is beginning to look like levels of vaccination above 85 to 90 per cent will be closer to our expectations of substantially reduced transmission. This might be difficult to achieve in some places with obstacles such as vaccine hesitancy, but it should remain our target,” St John said.
Achieving herd immunity would depend on the entire susceptible population considering vaccination. This should include children, who might show less severe outcomes, but could still pass on the infection. But most countries start their vaccination campaign with orders for health care workers, the elderly and older adults, according to Penelope Ward, fellow of the Faculty of Pharmaceutical Medicine and visiting professor in pharmaceutical medicine at King‘s College London. In many places, younger people have yet to become eligible for vaccination, making targets hard to reach, Ward noted. “Children under the age of 12 have not been included in the vaccine programmes in most countries and vaccination uptake in teenagers has been variable. Therefore, no country has yet achieved the immunity threshold that might reduce spread of infection within the community at large, even for the original strain, let alone for more infectious strains such as Delta,” she explained. Where children under 12 account for 15 per cent or more of the populace, herd immunity cannot be achieved until enough of that group are either vaccinated or have become immune due to infection, she added. In China, at least 91 per cent of schoolchildren aged 12 to 17 have been vaccinated. Children aged three to 11 years old have also become eligible for vaccination and are being offered doses in more than a dozen provinces at the moment. This covers about a fifth of the population, as China’s national census last year found children under 15 accounted for about 18 per cent of total citizens.
However, the combination of a more contagious variant and waning immunity is making achieving herd immunity more difficult, Ward said. The primary immunisation drive in China relied mostly on two inactivated vaccines developed by Sinopharm and Sinovac. Real-world effectiveness studies showed that they could still prevent serious illness and death, but gave limited protection to the elderly as antibody levels dropped rapidly. In September, a World Health Organization panel recommended that people over 60 vaccinated with Sinovac or Sinopharm vaccines should receive a third dose of any brand to increase protection. Some countries where these vaccines have been used for primary immunisation, such as the UAE and Brazil, have offered viral vector or mRNA vaccines as booster shots. China has offered booster shots of the same technology for the elderly and people whose jobs put them at high risk of catching or spreading Covid-19. Scientists said antibody levels drop again six months after the third dose, but it is unclear at the moment whether China will need another round of boosters. “More is not always better with vaccines and sometimes the memory response is best if you allow for the right timing between boosts,” Professor St John at NUS said. Wang Huaqing, a vaccination expert with the Chinese Centre for Disease Control and Prevention, said the country would need to deploy better vaccines to avoid repeated inoculation cycles. “Ideally we hope a vaccine will be very effective after full immunisation [regime], and even if a booster is needed later, the number of booster doses is limited. So we hope that in the future there will be better vaccines and better vaccination procedures to consolidate the population protection effect,” Wang told reporters last month. “Constantly boosting immunisation is not our ultimate choice.” However, even without reaching herd immunity, some countries that hitherto adopted a strict zero-tolerance strategy for Covid-19 like China have chosen a different direction and set forth border opening plans.
New Zealand pursued a zero-Covid strategy for nearly two years but last month set a target for easing restrictions once 90 per cent of the eligible population aged above 12 was fully vaccinated. Borders will open for some Pacific Island states without quarantine from this month and entry for arrivals from other low-risk countries will be considered early next year. Next door, Australia has taken steps to gradually reopen its borders after inoculating nearly 80 per cent of the eligible population. Singapore, once also in the club of nations following an elimination strategy, has decided to switch gears to “living with Covid”, allowing Covid-19 to become endemic once more than 77 per cent of the city state’s population was vaccinated in August.However, the combination of a more contagious variant and waning immunity is making achieving herd immunity more difficult, Ward said. The primary immunisation drive in China relied mostly on two inactivated vaccines developed by Sinopharm and Sinovac. Real-world effectiveness studies showed that they could still prevent serious illness and death, but gave limited protection to the elderly as antibody levels dropped rapidly. In September, a World Health Organization panel recommended that people over 60 vaccinated with Sinovac or Sinopharm vaccines should receive a third dose of any brand to increase protection. Some countries where these vaccines have been used for primary immunisation, such as the UAE and Brazil, have offered viral vector or mRNA vaccines as booster shots. China has offered booster shots of the same technology for the elderly and people whose jobs put them at high risk of catching or spreading Covid-19. Scientists said antibody levels drop again six months after the third dose, but it is unclear at the moment whether China will need another round of boosters. “More is not always better with vaccines and sometimes the memory response is best if you allow for the right timing between boosts,” Professor St John at NUS said.
Wang Huaqing, a vaccination expert with the Chinese Centre for Disease Control and Prevention, said the country would need to deploy better vaccines to avoid repeated inoculation cycles. “Ideally we hope a vaccine will be very effective after full immunisation [regime], and even if a booster is needed later, the number of booster doses is limited. So we hope that in the future there will be better vaccines and better vaccination procedures to consolidate the population protection effect,” Wang told reporters last month. “Constantly boosting immunisation is not our ultimate choice.” However, even without reaching herd immunity, some countries that hitherto adopted a strict zero-tolerance strategy for Covid-19 like China have chosen a different direction and set forth border opening plans. New Zealand pursued a zero-Covid strategy for nearly two years but last month set a target for easing restrictions once 90 per cent of the eligible population aged above 12 was fully vaccinated. Borders will open for some Pacific Island states without quarantine from this month and entry for arrivals from other low-risk countries will be considered early next year. Next door, Australia has taken steps to gradually reopen its borders after inoculating nearly 80 per cent of the eligible population. Singapore, once also in the club of nations following an elimination strategy, has decided to switch gears to “living with Covid”, allowing Covid-19 to become endemic once more than 77 per cent of the city state’s population was vaccinated in August.
Source: SCMP Expat Focus
