Two doses of the Chinese COVID vaccine Sinopharm produced nothing or only minimally detectable neutralising activity against some Omicron sub-variants, according to a small study published in The Lancet Infectious Diseases journal.

A third booster dose, however, partly restored the neutralising activity, revealed the study led by Chinese researchers from Anhui Medical University in Hefei and Beijing Institute of Microbiology and Epidemiology.

The study analysed 25 individuals who received two doses of the BBIBP-CorV vaccine. They found that the neutralising activity against Omicron sub-variants such as BA.1, BA.2, BA.2.11, BA.2.12.1, BA.2.13, and BA.4/BA.5 "was not or only minimally detectable".

Neutralising activity against these sub-variants was observed in 24-48 per cent of people who received a BBIBP-CorV booster post a two-dose shot. The neutralising activity saw a slight improvement to 30-53 per cent among 30 participants who received the third dose of another COVID vaccine ZF2001, developed by Anhui Zhifei Longcom.

Further, the findings showed that the BA.2.12.1 subvariant showed significantly more resistance than the BA.2 subvariant to a BBIBP-CorV booster, and the BA.2.11, BA.2.12.1, and BA.2.13 sub-variants showed significantly more resistance than the BA.2 subvariant to a ZF2001 booster.

The serum neutralising antibody titres against all tested pseudoviruses did not differ between people who received a BBIBP-CorV booster and those who received a ZF2001 booster.

About 18 people had BA.1 breakthrough infection and 15 people had BA.2.2 breakthrough infection. In addition, neutralising antibodies against omicron sub-variants above the limit of detection accounted for 88-100 per cent of infections, the study showed.