Recombinant African swine fever strains spreading across northern and central Vietnam, study finds

A scientific report published this month warns that recombinant African swine fever (ASF) viruses combining elements of genotype I and II have become widespread in laboratory-confirmed cases across northern and central Vietnam and are rising in frequency in diagnostic samples. The findings raise concerns that existing genotype II–based vaccines may offer limited protection against these mosaic strains and underscore the need for expanded surveillance and updated immunization strategies.

Key findings

• Researchers analyzed 182 clinical samples (spleen, lymph node, lung tissue and whole blood) collected from pigs suspected of ASF infection across 26 provinces in northern and central Vietnam between January 2023 and August 2024. All samples tested positive for ASF virus DNA by real-time PCR.

• Sequencing of viral gene regions showed that 73.08% (133 of 182) of samples carried genotype II, the form that has dominated Asian outbreaks since the late 2010s. The remaining 26.92% (49 of 182) carried a p72 gene associated with genotype I paired with other genetic segments characteristic of genotype II and a different serogroup — a pattern matching recombinant genotype I/II ASF viruses (rASFV I/II).

• Recombinant strains were detected in 17 of the 26 provinces sampled. The earliest recombinant in the dataset was a May 2023 sample from Bac Giang province, earlier than previously documented detections in published reports.

• The proportion of rASFV I/II in diagnostic submissions increased sharply in the study sample set: 14.14% (14 of 99) in 2023 to 42.17% (35 of 83) in 2024. The authors report a statistically significant rise (chi-square = 17.7; p < 0.001).

• Genetic diversity among recombinants is notable: within the B602L central variable region the team identified 13 distinct sequence variants among recombinant I/II viruses compared with two variants among genotype II viruses in the sample, indicating diversification atop a common recombinant backbone.

Vaccine and control implications

Vietnam has been among the earliest countries to license and deploy commercial live‑attenuated ASF vaccines derived from genotype II viruses, rolling out several products under emergency or conditional approvals since 2023. The study highlights a critical limitation of that approach: protection is largely strain-specific.

The authors note that two licensed live‑attenuated genotype II vaccines in Vietnam “protect mainly against homologous strains and are ineffective against rASFV I/II,” citing prior experimental work. Separate challenge studies from China published in 2023 similarly reported that a genotype II live attenuated vaccine did not protect pigs challenged with recombinant I/II viruses and that those recombinant viruses were highly lethal under experimental conditions.

Taken together, the evidence suggests recombinant I/II strains can both cause severe disease and evade some immunity induced by genotype II–based vaccines. That combination presents practical questions for animal health authorities, vaccine makers and farmers, particularly where genotype II vaccines are already in use and exported regionally.

Limitations of the study

The authors emphasize important caveats. Samples were not collected through randomized surveillance but were diagnostic submissions from farms and local veterinarians seeking confirmation, so the dataset may be biased toward more severe or unusual outbreaks. Because of that, the study cannot provide a nationwide prevalence estimate and its sample proportions should not be interpreted as direct measures of national incidence.

The report does not include data on numbers of pigs culled, outbreak sizes, economic losses, or how often disease occurred in vaccinated versus unvaccinated herds — leaving unanswered questions about real‑world vaccine effectiveness against these recombinants.

What authorities and experts say

As of the article’s early release, Vietnam’s Ministry of Agriculture and Rural Development and its Department of Animal Health had not issued public statements directly responding to the new findings. The study’s authors call for expanded genomic surveillance, improved diagnostics that can distinguish genotypes and recombinants, and systematic evaluation and updating of vaccines.

Economic and regional stakes

African swine fever does not infect humans but is often fatal to pigs and has in the past caused massive herd losses and supply shocks in China, Vietnam and elsewhere in the region. Pork is Vietnam’s main source of meat and a critical livelihood for millions of smallholders; renewed or prolonged ASF outbreaks can push up consumer prices and strain household incomes in rural areas.

If recombinant strains continue to spread and current genotype II vaccines offer limited protection, farmers may face renewed losses and higher biosecurity costs, and authorities may have to rely more on movement controls and culling. For trading partners and regional vaccine markets, questions about strain coverage and surveillance could influence import and regulatory decisions.

What’s next

The study provides a genetic snapshot — an early warning rather than a full accounting. The authors and outside experts say the priority should be broader, systematic genomic surveillance across the pig sector, better diagnostic tools to detect and monitor recombinants, and field evaluations of vaccine performance against emerging variants. How quickly veterinary services and vaccine makers adapt will help determine how exposed Vietnam’s pork sector — and regional meat markets — will be in the years ahead.

Acknowledgments: The paper is an early‑release dispatch in Emerging Infectious Diseases and was authored by scientists from Vietnam, South Korea and Canada. The report’s data cover samples collected through August 2024.