One antiviral actually worked: ensitrelvir cut household transmission by two-thirds
The strongest evidence for post-exposure prophylaxis comes from a 2026 trial of ensitrelvir, an oral antiviral. In that study, household contacts who took ensitrelvir within 72 hours of the index patient's symptoms had a COVID-19 rate of 2.9% versus 9.0% in the placebo group—a 67% relative risk reduction [3]. That means for every 100 people exposed, about 6 fewer developed symptomatic COVID-19. The trial was large (over 2,000 participants) and double-blind, so the result is robust [3].
But the other two major antivirals failed to show a significant benefit
In contrast, a 2024 trial of nirmatrelvir-ritonavir (Paxlovid) found no significant reduction in symptomatic infection: 2.6% in the 5-day group and 2.4% in the 10-day group versus 3.9% in placebo—differences that were not statistically significant [1]. Similarly, a 2023 trial of molnupiravir showed COVID-19 rates of 6.5% versus 8.5% in placebo, which also missed the prespecified threshold for significance [2]. Both trials were large and well-designed, so the lack of effect is unlikely to be a fluke.
Why did one drug work and the others not? The likely answer is immunity and timing
The molnupiravir trial enrolled unvaccinated adults, yet 83.6% already had antibodies from prior infection—so most participants were already protected, making it hard to show a benefit [2]. The nirmatrelvir trial also enrolled a highly immune population, and a modeling study suggested that the drug's in vivo potency is lower than expected, and that early treatment may blunt the immune response without fully clearing the virus [4]. Ensitrelvir, on the other hand, was tested in a more general population and started within 72 hours of the index patient's symptoms, which may have been a key factor [3]. So the evidence suggests that post-exposure prophylaxis is only useful when the exposed person is truly susceptible and the drug is started very early.
About These Sources
This answer is built on 5 peer-reviewed studies — published from 2022 to 2026, 3 from 2024 or later, 4 in Q1 journals, collectively cited 67 times — selected as the most relevant from 5 studies that passed quality screening, drawn from 60 papers retrieved from a database of over 500 million.
Sources used in this answer
Oral Nirmatrelvir–Ritonavir as Postexposure Prophylaxis for Covid-19
In a phase 2-3 double-blind trial of 2,736 household contacts, nirmatrelvir-ritonavir (5 or 10 days) did not significantly reduce symptomatic COVID-19 (2.6% and 2.4% vs. 3.9% placebo), with risk reductions of ~30-35% that were not statistically significant.
Molnupiravir for intra-household prevention of COVID-19: The MOVe-AHEAD randomized, placebo-controlled trial
In the MOVe-AHEAD phase 3 trial of 1,527 unvaccinated household contacts, molnupiravir did not meet the prespecified superiority criterion (COVID-19 rates 6.5% vs. 8.5% placebo), and 83.6% of participants had pre-existing antibodies.
Ensitrelvir for Covid-19 Postexposure Prophylaxis in Household Contacts.
In a double-blind, randomized, placebo-controlled trial of 2,041 household contacts, ensitrelvir significantly reduced COVID-19 incidence (2.9% vs. 9.0%, risk ratio 0.33, P<0.001) when started within 72 hours of the index patient's symptom onset.
A unifying model to explain frequent SARS-CoV-2 rebound after nirmatrelvir treatment and limited prophylactic efficacy
A mathematical model of nirmatrelvir treatment suggests its in vivo potency is lower than in vitro predictions, and that early treatment preserves susceptible cells and blunts innate immunity, explaining limited prophylactic efficacy and frequent viral rebound; extending treatment to 10 days may reduce rebound.
Assessing the Combined Public Health Impact of Pharmaceutical Interventions on Pandemic Transmission and Mortality: An Example in SARS CoV‐2
An agent-based model of the US pandemic (2020-2021) projected that monoclonal antibodies used as post-exposure prophylaxis could avert up to 14% more infections and 37% more deaths when added to vaccines, but this was during a period when variants were susceptible to those antibodies.