Seasonal influenza is common between January to March, and July to August in Hong Kong.¹Centre for Health Protection, Department of Health – Seasonal Influenza. Available at: https://www.chp.gov.hk/en/healthtopics/content/24/29.html (Accessed 9 December 2020). Baloxavir marboxil is the first influenza antiviral with a novel mechanism of action to be approved in almost two decades, and was registered for use in Hong Kong in February 2019.^2,3O’Hanlon R et al. Curr Opin Virol. 2019;35:14-18.
Drug Office, Department of Health. Available at: https://www.drugoffice.gov.hk/eps/drug/productDetail/en/consumer/122421 (Accessed 9 December 2020). It is approved for the treatment of acute uncomplicated influenza in patients 12 years of age and older, who have been symptomatic for no more than 48 hours.²O’Hanlon R et al. Curr Opin Virol. 2019;35:14-18 It was recently approved as post-exposure prophylaxis for influenza by both the US FDA and EMA, based on the results of the BLOCKSTONE study.^4-6 Roche announces FDA approval of Xofluza for the prevention of influenza following contact with an infected person. Roche. Available at: https://www.roche.com/media/releases/med-cor-2020-11-24.htm (Accessed 9 December 2020).
EMA Panel Backs Xofluza for Influenza, Post-Exposure Prophylaxis. Medscape. Available at: https://www.medscape.com/viewarticle/941059 (Accessed 9 December 2020).
Ikematsu H et al. N Engl J Med. Published online July 8, 2020.

The multicentre, double-blind, randomised, placebo-controlled BLOCKSTONE trial evaluated the postexposure prophylactic efficacy of baloxavir marboxil in household contacts of index patients with confirmed influenza during the 2018-2019 flu season in Japan.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. Index patients were defined as the first documented case of influenza in a household, on basis of a positive rapid influenza diagnostic test.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. In total 752 participants were classified as asymptomatic close contacts who had lived in the same household as the index patient or at least 48 hours before enrolment and were randomised 1:1 to receive either baloxavir or matching placebo.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. The primary efficacy endpoint was laboratory-confirmed clinical influenza, and key secondary endpoints included RT-PCR confirmed illness, and RT-PCR confirmed influenza virus infection regardless of fever and symptoms.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. Risk ratios were adjusted for randomization factors (time from onset of illness, type of antiviral treatment of index patient, and age of the participant [continuous variable]).⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020.

The majority of participants (72.5%) received baloxavir or placebo within 24 hours after onset of illness in the index patient.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. The primary endpoint of laboratory-confirmed clinical influenza was met by 1.9% and 13.6% of participants receiving baloxavir and placebo, respectively (adjusted risk ratio [aRR]=0.14; 95% CI: 0.06-0.30; p<0.001).⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. Baloxavir prophylaxis led to a lower percentage of patients with RT-PCR confirmed illness than placebo (aRR=0.24; 95% CI: 0.15-0.38), and a lower percentage of patients with RT-PCR-confirmed influenza virus infection regardless of fever and symptoms (aRR=0.43; 95% CI: 0.32-0.58).⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020.

Adverse events (AEs) were reported in 22.2% of participants who received baloxavir, and 20.5% of those who received placebo.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020. The most common adverse events were headache, haematuria, pharyngitis, and increases in alanine aminotransferase levels.⁶Ikematsu H et al. N Engl J Med. Published online July 8, 2020.