- 14 October 2021
China’s COVID vaccines have been crucial — now immunity is waning
Billions of shots of China’s CoronaVac and Sinopharm vaccines have been given globally, but studies have questioned the length of protection they offer.
China’s CoronaVac and Sinopharm vaccines account for almost half of the 7.3 billion COVID-19 vaccine doses delivered globally, and have been enormously important in fighting the pandemic, particularly in less wealthy nations.
But as the doses mount, so have the data, with studies suggesting that the immunity from two doses of either vaccine wanes rapidly, and the protection offered to older people is limited. This week the World Health Organization announced advice from its Strategic Advisory Group of Experts on Immunization (SAGE) that people over 60 should receive a third dose of the same or another vaccine to ensure sufficient protection.
The recommendation is “sensible and necessary”, says Manoel Barral-Netto, an immunologist at the Oswaldo Cruz Foundation in Salvador, Brazil.
CoronaVac, produced by Beijing-based company Sinovac, is the world’s most widely used COVID-19 vaccine. Not far behind is the vaccine developed in Beijing by state-owned Sinopharm (see ‘The race to vaccinate’).
In mid-2021, the World Health Organization (WHO) approved the shots for emergency use, on the basis of limited clinical-trial data suggesting that CoronaVac was 51% and Sinopharm 79% effective at preventing symptomatic disease. This was on a par with the 63% efficacy reported for the University of Oxford–AstraZeneca’s viral-vector vaccine at the time of its WHO listing, but lower than the 90% and higher efficacies of the mRNA vaccines developed by Pfizer–BioNTech and Moderna.
Both the Chinese vaccines are inactivated vaccines, which use killed SARS-CoV-2 virus. Researchers say this type of vaccine seems to be less potent because it triggers an immune response against many viral proteins. By contrast, mRNA and viral-vector vaccines target the response to the spike protein, which is what the virus uses to enter human cells.
“You don’t choose the target with inactivated vaccines, you just throw in all these different antigens,” explains Jorge Kalil, a physician and immunologist at the University of São Paulo Medical School, Brazil.
Lower antibody responses
Some studies have found that compared with vaccines made using other technologies, China’s inactivated vaccines initially generate lower levels of ‘neutralizing’ or virus-blocking antibodies — considered a proxy for protection — and that these levels drop quickly over time.
But “waning of antibodies isn’t necessarily the same as waning of immune protection”, says Ben Cowling, an epidemiologist at the University of Hong Kong. He says that vaccines induce complex immune responses, including B cells and T cells, which might be more long lived than neutralizing antibodies.
So far, studies assessing protection over time are limited. But preliminary analysis of a mass-vaccination campaign with CoronaVac in Chile suggests a small but significant decline in efficacy against symptomatic disease, although protection against hospitalization remains high, says Eduardo Undurraga, a public-health researcher at the Pontifical Catholic University of Chile in Santiago.
Vaccines made using other technologies have seen a similar trend of waning antibodies and protection against infection, but more-robust protection against severe disease and death. But researchers say that because the Chinese inactivated vaccines start at a lower base of neutralizing antibodies, the protection they offer could drop faster than those with a stronger head start.
To boost, or not to boost
The less-potent immune response from inactivated vaccines also has implications for the protection they offer to older people. The immune system weakens with age and vaccines are generally less effective in older people, says Kang, but the effect seems to be more pronounced with the inactivated vaccines.
A massive analysis of some one million people who were hospitalized with COVID-19 in Brazil4 found that CoronaVac offered up to 60% protection against severe disease up to the age of 79 — not far off the 76% protection offered by the Oxford–AstraZeneca vaccine.
But the picture changes drastically in people over 80, says co-author Daniel Villela, an epidemiologist at the Oswaldo Cruz Foundation at Rio de Janeiro, Brazil. In that group, CoronaVac was only 30% effective at preventing severe disease and 45% effective against death, compared with 67% and 85%, respectively, for the Oxford–AstraZeneca jab.
Clinical-trial data from China6, not yet peer-reviewed, suggest that a third dose of CoronaVac increases neutralizing antibody levels, and a similar boost has been observed in studies of third doses of Sinopharm’s vaccine.
And earlier this month, the Chilean government reported preliminary results on the effectiveness of booster shots, based on data from some two million people who had received two shots of CoronaVac, and a third shot of the CoronaVac, Pfizer–BioNTech or Oxford–AstraZeneca vaccines. Protection against COVID-19 jumped from 56% after two shots to 80% or higher after a third shot of any vaccine, with protection against hospitalization rising from 84% to 87%.
“For all vaccines, it’s very much an evolving situation,” says Kang. “Inactivated vaccines are a big part of our portfolio. So we really need to figure out how to use them.”