In the absence of Omicron-specific vaccines, extending the vaccination interval between doses may provide a higher level of protection against Omicron sub-variants, according to a new study.

The study, published in The New England Journal of Medicine earlier this month, was led by Gao Fu, an academician of the Chinese Academy of Sciences (CAS) and head of the Chinese Center for Disease Control and Prevention (CDC).

Researchers in the study assessed the neutralization antibody levels against the SARS-CoV-2 original and Omicron sub-variants including BA.4 and BA.5 in blood samples obtained from vaccine takers, who had received three doses of China-made inactivated vaccines, three-shot protein-subunit vaccine ZF2001, or two doses of inactivated vaccines boosted by ZF2001.

Results showed that in each vaccine group, the neutralizing antibody levels against all the tested Omicron sub-variants were significantly lower than the levels against the original strain of the virus, indicating these subvariants have an ability to evade immune protection.

But for the ZF2001, researchers found that neutralizing antibody levels rose with the increasing interval between the second and third doses, especially against the Omicron sub-variants.

For vaccine takers who had an interval of four to six months between the second and third doses, their neutralizing antibody levels were higher by nearly a factor of 10 against the original variant and by a factor of approximately 30 against all Omicron sub-variants, as compared with those who had a 1-month interval between doses.

Researchers explained that the better performance of the ZF2001 was due to its use of a receptor-binding domain as the antigen, which could induce increased levels of neutralizing antibodies against Omicron sub-variants through the administration of multiple booster doses.

The ZF2001 was jointly developed by the Institute of Microbiology under the CAS and Anhui Zhifei Longcom Biopharmaceutical Co. Ltd. China granted conditional market approval to the vaccine in March.

The Omicron sub-variant BA.5 is becoming the dominant strain of COVID-19 across the world and is causing cluster infections in China. Regions including Beijing, Tianjin and Shaanxi have recently reported local COVID-19 cases of the Omicron sub-variant arriving from overseas.

For better protection against immune escape of current and possible future epidemic sub-variants, the development of updated vaccines as boosters is still needed, according to the study.