In the absence of the Omicron-specific vaccines, extending the vaccination period between doses may contribute to stopping the rapid emergence of this COVID-19 mutation, said a new study.

Published in The New England Journal of Medicine earlier this month, the study 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.

Researchers in the study assessed the neutralization levels against the SARS-CoV-2 original and Omicron subvariants 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 subvariants 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 subvariants.

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 subvariants, 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 subvariants 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 subvariant 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 subvariant arriving from overseas.

According to the study, for better protection against immune escape of current and possible future epidemic subvariants, the development of updated vaccines as boosters is still needed. 

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