COVID-19 vaccines for the older population: safety and efficacy go hand in hand.

The increased infectivity and immune escape capabilities of the SARS-CoV-2 omicron variant (B.1.1.529) have heightened concerns surrounding the spread of COVID-19. Compared with other SARS-CoV-2 variants, omicron causes less severe disease in people aged 20–59 years, but the risk of serious illness and death remains high in people aged 60 years or older, particularly in those with underlying diseases. Thus, risk in the older population must be reduced via vaccination with an effective and safe COVID-19 vaccine.

In The Lancet Healthy Longevity, Eric Yuk Fai Wan and colleagues report a modified self-controlled case series on the total and individual incidences of 30 adverse events of special interest in the inpatient setting within 21 days (2 days for anaphylaxis) of the first, second, or third dose of CoronaVac (Sinovac Biotech), an inactivated SARS-CoV-2 vaccine. More than 1·2 million people aged 60 years or older were recruited in Hong Kong. Adverse events of special interest were selected from the list of events recommended by the WHO Global Advisory Committee on Vaccine Safety and included acute disseminated encephalomyelitis, microangiopathy, heart failure, and acute liver injury. The authors found that the incidence of adverse events of special interest in this population ranged from 0·00 per 100 000 people to 57·49 per 100 000 people. No significant increased risk of an adverse event of special interest was observed after the first or third dose of CoronaVac, but a significantly higher risk of anaphylaxis within 2 days of the second dose was reported (compared with a baseline period). These results indicate that CoronaVac has a good safety profile in the older population, which might help to alleviate vaccine hesitancy due to concerns of adverse events associated with COVID-19 vaccines.

CoronaVac is likely to be one of the safest COVID-19 vaccines based on all the cumulated safety data. However, attention should be paid to the efficacy of the vaccine as well as to its safety. A study in Brazil conducted between Feb 24, 2020, and Nov 11, 2021, during which time the gamma (P.1) SARS-CoV-2 variant was dominant, involved mainly people younger than 45 years with previous SARS-CoV-2 infection and showed that two doses of CoronaVac were 81·3% (95% CI 75·3–85·8) effective against hospitalisation or death 14 days or more after vaccination; however, this study also showed no major differences in effectiveness between adults aged 18–49 years versus those aged 50 years or older. In another study in Brazil that recruited older adults (aged ≥70 years) from Jan 17 to April 29, 2021, when the gamma variant was dominant, the effectiveness of the second dose of CoronaVac 14 days after vaccination was 46·8% (95% CI 38·7–53·8) against symptomatic COVID-19, 55·5% (46·5–62·9) against hospitalisation, and 61·2% (48·9–70·5) against death; moreover, increasing age was shown to be associated with reduced vaccine effectiveness, a disparity that could be worsened when the omicron variant is dominant. The concern is that the impact of omicron's high infectivity and immune escape capacity could exceed that of previous variants, such as alpha (B.1.1.7), delta (B.1.617.2), and gamma, posing a greater challenge to the effectiveness of CoronaVac. In a study done in Hong Kong, the two-dose regimen of CoronaVac was found to induce neutralising antibodies against wild-type SARS-CoV-2 in 24 (86%) of 28 adults with a median age of 29 years (IQR 26–31). However, two doses of CoronaVac induced neutralising antibodies in only 14 (50%) adults for the delta variant and in only two (7%) adults for the omicron variant.

On April 26, 2022, Sinovac Biotech's new inactivated COVID-19 vaccine based on the omicron variant was officially approved by the State Drug Administration of China to enter clinical studies. One clinical study (NCT05381350) has been launched in China to evaluate immunogenicity against the SARS-CoV-2 omicron variant induced by a one-dose booster with Sinovac Biotech's new inactivated COVID-19 vaccine in people who have been primed with two or three doses of other COVID-19 vaccines based on wild-type SARS-CoV-2.

It is encouraging that the results of Wan and colleagues' study support the safety of CoronaVac in older adults, which might help to relieve vaccine hesitancy. Ideal vaccines should have good safety and high efficacy and we should continue our efforts in their production.

We declare no competing interests.