Four COVID-19 booster doses were the most effective way to prevent critical Omicron BA.5, regardless of previous infection status, according to a nationwide study published late last week in JAMA Network Open.
South Korean researchers used the Korea COVID-19 Vaccine Effectiveness data set to estimate the protection conferred by vaccination and previous Omicron BA.1 or BA.2 infection against BA.5 infection and critical illness among 3,415,980 who tested positive for the virus and an equal number of matched controls from Aug 1 to 31, 2022. The average age was 40.2 years, and 53.9% were female.
2-dose VE low in all groups
Vaccine effectiveness (VE) of four doses of COVID-19 vaccine against BA.5 infection among COVID-naïve participants was 16.1%, while it was 89.5% and 94.3% among those previously infected with BA.1 and BA.2, respectively.
VE of two doses against critical BA.5 was low in all groups, including COVID-naïve participants (41.5%) and those previously infected with BA.1 (53.1%) and BA.2 (50.0%). But four-dose VE against critical BA.5 was 90.9% among COVID-naïve participants and 93.9% and 92.9% among those with previous BA.1 and BA.2 cases, respectively.
The observation that the protection against critical BA.5 infection depends more on 4-dose booster, rather than previous BA.1 or BA.2 infection, highlights the importance of booster vaccination.
The authors cautioned that some infections may have been missed, and protection from previous infection could have been overestimated if exposed participants sought testing more often than their unexposed peers.
"Importantly, the observation that the protection against critical BA.5 infection depends more on 4-dose booster, rather than previous BA.1 or BA.2 infection, highlights the importance of booster vaccination," they concluded.