Canadian data show moderate to good vaccine efficacy against COVID, flu

Coughing girl

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The Canadian Sentinel Practitioner Surveillance Network (SPSN) data reveal mid-season vaccine effectiveness (VE) against the SARS-CoV-2 Omicron XBB.1.5 variant of 47% against medically attended outpatient COVID-19 and 67% among previously infected people.

The same test-negative case-control study reports that the flu vaccine is 63% effective against medically attended outpatient infection with the influenza A H1N1 strain and 40% against H3N2.

The researchers enrolled patients with new or worsening cough that is characteristic of acute respiratory illness (ARI) who sought care within 7 days of symptom onset from community-based sentinel practitioners in Alberta, British Columbia (BC), Ontario, or Quebec.

The team also evaluated patients with influenza-like illness (ILI), a diagnosis requiring fever and sore throat, joint or muscle pain, and extreme weakness or exhaustion in addition to cough, for comparison with previous SPSN analyses; patients aged 65 years and older didn't have to have fever for the diagnosis. Specimens were obtained from October 29, 2023, to January 13, 2024.

The results were published last week in Eurosurveillance.

Vaccine halved risk of outpatient COVID

Of 2,176 respiratory specimens obtained from patients aged 12 years and older in BC, Ontario, and Quebec, 15% tested positive for COVID-19. Researchers used whole-genome sequencing on 236, identifying the SARS-CoV-2 Omicron variants EG.5.1 (15%), HV.1 (27%), and JN.1 (38%). The proportion of JN.1 rose from 11% in weeks 44 to 47 to 34% to 81% in the last 3 weeks of the study.

Among individuals reporting a prior confirmed SARS-CoV-2 infection, XBB.1.5 [vaccine] provided higher protection, reducing the COVID-19 risk by around 70% overall.

VE against medically attended COVID-19 was estimated at 47%. VE among the 61% of participants who previously tested positive for COVID-19 was 67%.

"The updated autumn 2023 monovalent XBB.1.5 vaccine afforded a level of protection comparable to the seasonal influenza vaccine, reducing the risk of medically attended outpatient COVID-19 by about 50% overall," the study authors wrote. "Among individuals reporting a prior confirmed SARS-CoV-2 infection, XBB.1.5 provided higher protection, reducing the COVID-19 risk by around 70% overall."

Vaccine tied to 60% lower H1N1 risk

Of the 3,139 respiratory specimens collected, 24% tested positive for flu, 94% of which were influenza A and 6% were influenza B. Among the 696 subtyped influenza A viruses, 80% were H1N1, and 20% were H3N2. A total of 382 H1N1 viruses were characterized by hemagglutinin; 51% of those were vaccine-matched clade 5a.2a.1, and 49% were clade 5a.2a.

Adjusted flu VE was 63% against H1N1 and 40% against H3N2. H1N1 VE was lower against vaccine-matched clade 5a.2a.1 viruses (56%) than against clade 5a.2a viruses (67%). Among participants 12 years and older in BC, Ontario, and Quebec, VE was comparable against H1N1 (63%), H3N2 (43%), and H1N1 clades 5a.2a.1 (51%) and 5a.2a (73%).

"From late October 2023 through mid-January 2024, the Canadian SPSN estimates that the 2023/24 multivalent influenza vaccine reduced the risk of medically attended outpatient ARI due to influenza A(H1N1)pdm09 by about 60% and the risk due to influenza A(H3N2) by 40%," the researchers wrote. They noted that the 40% against H3N2 is consistent with previous VE estimates against that type.

Thus far this season, all analyses of XBB.1.5 and flu VE have spanned just 1 or 2 months post-vaccination, and Omicron variants continue to diversify, warranting continued monitoring, the authors noted.

"While BA.2.86 harbours more than 30 spike mutations relative to XBB.1.5, evidence so far suggests that immune evasion is not substantially greater than other variants," they wrote. "Compared with its parent BA.2.86 lineage, however, JN.1 carries an additional mutation that enhances antigenic distinction and the capacity for antibody escape."


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