The study incorporated data from August 2021 to January 2022, beginning when Delta was the dominant SARS-Cov-2 strain in the US and into the period of Omicron dominance.
The researchers confirmed that vaccine effectiveness against both hospitalisation and emergency department (ED) admission was higher after three doses than after two doses, but protection from both two and three doses began to wane with increasing time since vaccination, regardless of which variant was dominant.
How does vaccine effectiveness hold up against Omicron?
Overall, vaccine effectiveness against Omicron appeared to be lower than that against Delta. During the period of Delta dominance, vaccine effectiveness against ED admission with three doses was 97% within two months of vaccination and dropped to 89% for those vaccinated four or more months ago. Similarly, effectiveness against hospitalisation was reduced from 96% within two months of vaccination to 76% after four months.
By contrast, during the Omicron-dominant period, effectiveness of three vaccine doses against ED admission was measured at 87% within two months of vaccination and only 66% four to five months post-vaccination. Effectiveness of three doses against hospitalisation was 91% among those vaccinated in the past two months, declining to 78% after four months.
Should we be worried?
While that may sound concerning, Australian experts’ reactions to the report were generally positive.
“The excellent news from this study is that even more than four months after a booster dose, efficacy against hospitalisation was still nearly 80%, indicating that the booster doses are continuing to provide protection over a prolonged period,” said Deborah Cromer, senior research fellow at the Kirby Institute at the University of New South Wales.
“The data presented by the CDC actually shows how effective our current vaccines are, even against the Omicron variant, particularly in the first 2-3 months following the booster,” agreed Daniel Layton, an immunologist from the CSIRO.
Because the study specifically tracked hospitalisations and ED admissions, it did not include data about people with COVID who did not attend a hospital. The study also did not separate people by age group, underlying health status or vaccine brand, so it could not comment on the impact of these factors on vaccine protection.
From a public health perspective, however, these underlying patient factors do influence vaccination strategies. A fourth dose is already recommended for severely immunocompromised Australians.
“A major problem of opening borders and the ‘let it rip’ approach is that although infection with Omicron is milder for most people, it can still cause serious disease in the elderly and those with underlying health conditions such as diabetes,” said Adrian Esterman, chair of biostatisics at the University of South Australia.
“Perhaps it is now time for ATAGI to widen the group of people able to receive a fourth dose to this at-risk group, especially since many of them received their third dose two to three months ago.”
The future of COVID vaccines
New Omicron-specific vaccines currently in development by Pfizer and Moderna have the potential to further boost protection against Omicron in the second half of 2022. According to Esterman, such variant-specific vaccines are a promising option for fourth doses in the general population.
“Hopefully, the combination of natural immunity from these Omicron waves and lingering booster immunity, in combination with common-sense COVID-safe measures, will protect us till then,” said Sanjaya Senanayake, an infectious disease specialist at the Australian National University.
Senanayake also cited intranasal vaccines currently in clinical trials as an exciting development.
“These vaccines may be more effective at providing robust immunity in the nasopharynx itself and therefore make it harder for a recipient to get infected and transmit the virus,” he said.
While waning effectiveness may sound discouraging, the experts didn’t see the CDC report as a reason to delay a third dose or distrust vaccine protection.
“There is no guarantee that a future variant will not be more virulent than Omicron,” warned Senanayake.