Covid antibodies offer weak defence against fast-spreading BA.5 sub-variant

Elderly people arrive for Covid-19 vaccination at Bang Sue Grand Station in Bangkok on Sunday as health authorities warn that the newly discovered BA.5 - now the dominant sub-variant of the disease - better eludes antibodies, making booster shots necessary for vulnerable groups of people. (Photo: Wichan Charoenkiatpakul)

Antibodies are less effective in protecting people from the stronger, now-dominant BA.5 sub-variant of Covid-19, according to the Department of Medical Sciences.

Dr Supakit Sirilak, director-general of the department, said on Monday the laboratory examination of blood from people who had received three doses of Covid-19 vaccines showed their antibodies were less effective in preventing infection with BA.5 compared with the BA.2 sub-variant.

Tests were conducted on people who had been jabbed with combinations of Sinovac/Sinovac/AstraZeneca, Sinovac/Sinovac/Pfizer, AstraZeneca/AstraZeneca/Pfizer and Sinovac/AstraZeneca/AstraZeneca shots in series.

"The results show that BA.5 is better at eluding antibodies than BA.2, and vaccines are less effective against it (BA.5). However, three doses of any mix of vaccines can prevent the disease fairly effectively if they were not administered too long ago. The prevention of severe symptoms is natural," Dr Supakit said.

"Therefore, vulnerable groups of people should receive booster shots if their third shot was long ago," he said.

According to Dr Supakit, BA.4 and BA.5 infections have already surpassed the BA.2 sub-variant in the country.

Of 468 samples examined from July 16 to 22, 320 cases (68.4%) contained BA.4/BA.5, 143 cases (30.6%) were of BA.2 and five cases were of the BA.1 sub-variant.

All the samples from new arrivals in the country contained BA.4/BA.5, as did about 80% of Covid-19 cases in Bangkok and about 60% in other provinces.

"BA.4/BA.5 is clearly highly transmissible, so new Covid-19 cases are rising," Dr Supakit said.

Initial examination did not separate BA.4 cases from BA.5. Whole genome sequencing showed that about 75% of them were BA.5 and about 25% BA.4.

In Bangkok, 77% of patients infected with Covid-19's Omicron variant, but with only mild symptoms, had BA.4/BA.5 while 23% had BA.2. Meanwhile, 87% of those with severe symptoms and those who died of the disease had BA.4/BA.5 while 13% had BA.2.

In other provinces BA.4/BA.5 accounted for 56% of Omicron cases with mild symptoms, and BA.2 42%. Meanwhile, BA.4/BA.5 were responsible for 74% of Omicron cases with severe illnesses and fatalities, and BA.2 26%.

The data indicated that BA.4/BA.5 were more severe than previously found sub-variants, Dr Supakit said.