Viral blips during HIV therapy are common and rarely signal treatment failure, Swedish study finds

For people living with HIV, modern antiretroviral therapy can suppress the virus to levels below what standard laboratory tests can detect. Yet some patients on long-term treatment experience a curious phenomenon: a single test result showing a small, transient amount of virus in the blood, followed by an immediate return to undetectable levels. These events, known as viral blips, have puzzled clinicians and researchers for years, raising questions about whether they represent harmless statistical noise, brief bursts of viral production from hidden reservoirs, or early warnings of impending treatment failure.

A new nationwide Swedish study, published in BMC Infectious Diseases, offers one of the most comprehensive answers to date. Drawing on the Swedish national InfCareHIV cohort, researchers led by Erik Sörstedt of the University of Gothenburg analyzed data from 4,210 treatment-naïve adults who began antiretroviral therapy from 1 January 2007 onward, with follow-up data extracted through 30 September 2020. The median observation time was 5.5 years, with some individuals followed for as long as 13.7 years. The sheer scale of the cohort allowed the team to examine blip incidence across different drug classes with unusual statistical precision.

The study defined a viral blip as a transient plasma HIV RNA level between 50 and 500 copies per milliliter, preceded and followed by a result below 50 copies per milliliter. This definition distinguishes blips from sustained low-level viremia, which may indicate developing drug resistance or inadequate drug penetration. Virological failure, by contrast, was defined as two consecutive HIV RNA samples above 200 copies per milliliter or a single sample exceeding 1,000 copies per milliliter, in previously suppressed individuals who had been on therapy for more than 12 months.

The results showed that viral blips are far from rare. In total, 853 individuals, or 20.3 percent of the cohort, had between one and six registered blips during follow-up. The overall blip incidence was 4.35 per 100 person-years, with a 95 percent confidence interval of 4.02 to 4.69. In a sensitivity analysis using a narrower definition of 50 to 199 copies per milliliter, the incidence was slightly lower at 3.80 per 100 person-years. These figures suggest that roughly one in five people on suppressive therapy will experience at least one blip over a five-year period.

A central question the researchers addressed was whether the third agent in a three-drug regimen influences blip risk. The team compared integrase strand transfer inhibitor (INSTI)-based regimens with those built around non-nucleoside reverse transcriptase inhibitors (NNRTIs) and protease inhibitors (PIs). INSTIs, which block the viral enzyme that inserts HIV DNA into the host genome, have become the backbone of first-line therapy in many countries due to their potency and tolerability. The analysis revealed that individuals on INSTIs and NNRTIs had similar blip risk, while those on PI-based regimens had a significantly higher risk, with a hazard ratio of 1.22 (95 percent confidence interval 1.03 to 1.45; p = 0.02).

This finding has pharmacological implications. Protease inhibitors require boosting with ritonavir or cobicistat to achieve adequate drug levels and are known to have more complex drug-drug interaction profiles than INSTIs. Whether the higher blip rate on PIs reflects differences in drug potency at sanctuary sites, variations in adherence patterns, or other factors remains unclear. The authors note that the pathogenesis of viral blips is not completely understood, and their study does not resolve the underlying mechanism. However, the observation that the third agent class matters suggests that regimen composition may influence the frequency of transient viremia below 500 copies per milliliter.

Perhaps the most clinically reassuring result concerns the relationship between blips and subsequent virological failure. Contrary to the hypothesis that blips might serve as early harbingers of treatment failure, the study found that individuals with blips had a lower risk of virological failure than those with continuously undetectable HIV RNA. The hazard ratio was 0.54 (95 percent confidence interval 0.41 to 0.71; p < 0.0001). This counterintuitive finding suggests that transient viremia below 500 copies per milliliter is not associated with a higher risk of subsequent failure in this cohort. The authors used Cox regression for event analyses, a standard statistical approach for time-to-event data that accounts for varying follow-up durations.

The lower failure risk among individuals with blips may reflect surveillance bias rather than a protective biological effect. Patients who experience a blip receive closer monitoring, and any developing resistance or adherence problem may be detected and addressed earlier. Alternatively, the immune systems of individuals who mount transient viral production may be more active in controlling the virus. The study does not distinguish between these possibilities, but the practical message is clear: a single low-level positive result in an otherwise suppressed patient should not automatically trigger regimen changes or alarm.

The findings come at a time when INSTI-based regimens, particularly those containing dolutegravir, have become the dominant first-line therapy worldwide. The Swedish cohort spans a period from 2007 to 2020, capturing the transition from older NNRTI- and PI-based regimens to modern INSTI-based combinations. This longitudinal perspective is valuable because it reflects real-world clinical practice rather than the controlled conditions of randomized trials, where adherence support and monitoring are typically more intensive.

For clinicians managing people with HIV, the study provides evidence-based reassurance that viral blips are common, largely benign events that do not predict treatment failure. For researchers, the observation that PI-based regimens carry a higher blip risk than INSTI- or NNRTI-based regimens opens new avenues for investigating how drug class influences residual viral production from latent reservoirs. As the authors acknowledge, the pathogenesis and clinical consequences of viral blips remain incompletely understood, but this large nationwide cohort brings the field closer to a coherent explanation of one of HIV medicine’s most persistent curiosities.

Subject of Research: Virology

Article Title: Understanding viral blips in people with HIV on antiretroviral therapy: a nationwide Swedish cohort study of 4,210 individuals

Article References: Sörstedt, E., Nilsson, S., Sönnerborg, A., Carlander, C., Treutiger, C. J., Månsson, F., Änghagen, L., Berggren, H., Gisslén, M., & Yilmaz, A. (2026). Understanding viral blips in people with HIV on antiretroviral therapy: a nationwide Swedish cohort study of 4,210 individuals. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14654-z

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14654-z

Keywords: Understanding, viral, blips, people, antiretroviral, therapy, nationwide, Swedish, cohort, individuals, scientific research