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Overall two-dose vaccine effectiveness (VE) against Omicron variant-related hospitalization or death remained above 70% for at least 6 months among adults in Hong Kong but dropped to below 62% in those aged 80 and older, necessitating a third dose, finds a case-control study published today in JAMA Network Open.

Hong Kong researchers used linked administrative data to estimate VE of the CoronaVac and Pfizer/BioNTech COVID-19 vaccines among 32,823 adults who died or were hospitalized due to Omicron from Jan 1 to Jun 5, 2022 (case participants) and 131,328 infected control participants listed in a public health registry during the same period. The average age in both groups was 65 years.

Greater than 90% VE after booster

Most (62.1%) of the 9,362 case participants who died and 49.5% of the 16,236 who died or were hospitalized were unvaccinated, compared with 37% of 37,488 and 21.4% of 131,328 controls, respectively.

VE against severe disease rose with the number of doses for both types of vaccine. The overall VE of CoronaVac against death climbed from 56.4% after one dose to 71.1% after two doses to 88.5% after three. VE of the Pfizer vaccine against death increased from 62.2% after one dose to 70.8% after two doses to 84.4% after the third.

For at least 6 months, VE against severe outcomes after a second dose of the CoronaVac and Pfizer vaccines was 74.0% and 77.4%, respectively. Two-dose VE against death in participants aged 18 to 49 years was 86.4% and 92.9% among CoronaVac and Pfizer vaccine recipients, respectively, but fell to a respective 61.4% and 52.7% in those aged 80 and older after 6 months.

A booster dose is recommended for older patients to restore immunity.

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Four to 6 months after the third dose, overall VE against death was greater than 90% for CoronaVac and Pfizer vaccines and among participants who received a combination of the two.

"A booster dose is recommended for older patients to restore immunity," the authors wrote. "This is especially critical in a setting like Hong Kong, where third-dose coverage is still insufficient among older residents."

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