Introduction
Frozen food doesn’t exactly scream “public health threat.” Yet over the last decade, viruses like Norovirus and Hepatitis A have hitched quiet rides on everything from strawberries to salad leaves, triggering outbreaks, recalls and nervous glances at the freezer aisle. We often hear about one-off incidents, a contaminated batch here, a spike in testing there – but what’s harder to see is the bigger picture: how often does this really happen? Is the testing reliable? And why do some countries seem to show up in alerts more than others?
This article explores what we do know (and what we often don’t) about viral contamination in the fresh and frozen food supply chain. We’ll look at where the risks are highest, how testing works (and doesn’t…), and why it’s still so hard to draw a clean line between a positive result and a meaningful public health threat.
The Viral Contaminants You Can’t Cook Off (Unless You Actually Cook Them)
While bacteria like Salmonella and Listeria are well known in food safety circles, viruses are the quieter villains, often overlooked but no less dangerous.
The three major viral threats in the food chain are[1]:
- Norovirus: The most common cause of foodborne illness worldwide. Highly contagious, with just a few particles enough to cause ‘explosive’ symptoms. Often linked to fresh produce, shellfish, and typically from infected food handlers.
- Hepatitis A: Less common but more serious. Causes liver inflammation, long recovery times, and sometimes long-term health effects. Often associated with produce grown close to the soil, food washed in contaminated water, or handled by infected workers. Also found in bivalve shellfish. Examples include leafy greens, frozen or fresh berries, clams, oysters, and undercooked meat or fish.
- Hepatitis E (emerging in Europe): Mostly linked to undercooked pork or game meat but has occasionally appeared in shellfish and salads. Can be particularly dangerous in pregnant women or immunocompromised people.
These viruses do not grow in food like bacteria, they don’t multiply. But they can survive harsh environments: freezing, refrigeration, low pH. Once on a food surface, they’re incredibly resilient and often undetectable without targeted testing. Because you can’t see, smell, or taste them, they pass easily through the food chain unless you’re actively looking.
Enteric viruses (those infecting the gut) are shed in enormous numbers in the faeces of infected people or animals (often billions of particles per gram). They don’t need to multiply in food to cause illness, just to survive long enough to be swallowed. Though many types of viruses are excreted, the ones that consistently cause issues in food are currently relatively limited.
The Viruses Everyone Does Talk About (but Rarely in Food)
COVID-19, avian influenza (bird flu), and swine flu aren’t classic foodborne viruses, but they’ve shaped how we talk about contamination, hygiene, and transmission risks.
- COVID-19 (SARS-CoV-2): At the start of the pandemic, concerns about surface contamination (including on food packaging) led to increased cleaning protocols, despite minimal evidence of transmission via food consumption. That said, it reminded us how long viruses can survive on surfaces.
- Avian influenza (H5N1) and swine flu (H1N1): These zoonotic viruses (transmitted between animals and humans) have implications for food safety, especially in live animal markets and processing plants. Occasionally, they prompt recalls and widespread culling.
These viruses aren’t lurking in salad bags or berry mixes, but they’ve raised public awareness of how viruses can move through food supply chains. They’ve also reinforced the link between human, animal, and environmental health.
The Quiet Risk of Foodborne Viruses
Fresh and frozen produce can carry viruses from the field to your fork. All it takes is one contaminated water source, one unhygienic food handler, or one missed cleaning step. Because the infectious dose of Norovirus and Hepatitis A is so low, even a trace amount can be enough to spark illness.
High-risk foods include:
- Soft berries (e.g. strawberries, raspberries, blueberries)
- Leafy greens
- Fresh herbs
- Shellfish (especially when raw)
And while a wide range of products could carry viral risks, recent data from both FDA and EU RASFF systems paints a broader picture. In 2023, the FDA listed nine virus-related food recalls, eight involving Hepatitis A in berries. However, EU data shows a different emphasis: all 12 RASFF virus alerts that year involved Norovirus in oysters. And in early 2024, of 11 virus-related alerts, 5 were oysters (Norovirus), 4 were berries (Norovirus) and 2 were berries (Hepatitis A).
This tells us two things: first, the issue isn’t limited to berries; and second, the overall number of serious alerts is relatively low. Virus presence in food is still a genuine concern, but one that’s easy to misinterpret without context or volume data. Of course these carry a significant and somewhat magnified risk as outbreaks are often reported by various in-country medical organisations. Many of these viruses may also be ‘atypical’ to the region where small or localised outbreaks occur which can trigger governmental activities and linked media articles.
In 2018, a multi-country outbreak of Listeria monocytogenes linked to frozen sweetcorn led to nine deaths across Europe, a sharp reminder that frozen doesn’t always mean safe[2]. Though Listeria is a bacterium, not a virus, the incident underlined a wider issue: people increasingly use frozen vegetables in smoothies and salads without cooking them. That same assumption (that freezing is a kill step) creates a similar risk profile for viruses like Norovirus and Hep A, which can survive cold storage just fine.
When Positive Doesn’t Mean Infectious
Testing for viruses in food usually relies on real-time RT-PCR methods that detect viral RNA. That means a “positive” result confirms the presence of genetic material, not necessarily live, infectious virus.
The virus may be dead, but its RNA lingers long enough to trigger alarms.
This has real-world consequences. A batch of berries testing positive for Hep A might be destroyed or recalled, even if the virus itself is no longer viable. Testing at specialist ISO17025-certified labs (like Campden BRI[3]) is highly controlled, but the distinction between detection and infectivity remains unresolved.
This also creates a loophole: foods that are infectious might test negative, especially if sampling misses a contaminated area. It’s a needle-in-a-haystack problem. As I often say in training courses: testing is not a control, it should be used as a monitor and guide.
Who Tests What – and Why It’s Potentially Political
Some countries test more than others. Some don’t test at all. Some issue recalls based on a single positive; others require confirmed infectivity.
- In the US, FDA surveillance from 2018–2020 tested over 1,500 frozen berry samples, with fewer than 1% returning positive for Hep A or Norovirus[4].
- In the EU, the RASFF system logs food safety alerts submitted by member states. A few countries (like Spain or Italy) appear to test more actively, leading to more detections[5].
- In Morocco, strawberries faced multiple RASFF alerts in early 2024, triggering investigations by ONSSA, Morocco’s food safety authority. But follow-up tests often returned negative, adding complexity to the narrative[6].
The testing landscape is a patchwork. Private labs find issues that never get reported. Retailers and wholesalers may reject shipments quietly, and national authorities vary in how (and whether) they share data.
More testing = more positives, not necessarily more risk.
Fewer alerts doesn’t always mean clean food, just less scrutiny.
How Reliable Are the Numbers?
We often quote prevalence stats like gospel, but they can rest on shaky ground:
- Most RASFF alerts don’t disclose sample sizes.
- Surveillance programs rarely publish test volumes or thresholds.
- Positives from private testing don’t enter the public record.
This makes it hard to benchmark risk or compare regions. When detection relies on composite sampling or minimal quantities, true prevalence remains murky.
What Can Producers and Importers Actually Do?
- Know your water: Irrigation water is a major contamination vector, especially when untreated or shared between farms. Mobile hygiene and sanitation stations (often used in picking and pack-house environments) also require close scrutiny.
- Audit for hygiene: Ensure food handlers, harvesting crews, and packhouse staff follow virus-aware protocols (handwashing, exclusion when ill, proper sick pay to prevent ‘forced’ working, glove changes, and clean sanitation access).
- Understand your lab report: RNA detection isn’t proof of infectious virus, but it’s still a red flag. Treat it as a call to action.
- Assess your risk: Some commodities, countries, or seasons require more vigilance. Keep laser focus on known high-risk regions and remember that migrant workforces often move with the harvest.
- Push for transparency: Ask suppliers for testing volumes, water safety data, and incident history. In microbiology, context is everything.
And underpinning all of this is your ESG responsibility.
Food safety and ethical sourcing are inseparable. Migrant workforces exist across the globe -from Kent to Spain to North Africa (and they travel with the crops). That means sanitation risks, training needs, and welfare issues can shift with the season.
The best protection? Strong, enforceable ethical policies, independent supplier audits, and active engagement with ESG risks, from labour conditions to water quality.
In today’s climate of global unrest, travel disruptions, and workforce volatility, ESG isn’t a “nice-to-have.” It’s a mandatory layer of food safety resilience.
Final Thought: What “Clean” Actually Means
A negative test doesn’t guarantee safety. A positive test doesn’t guarantee danger. Somewhere in between lies the uncomfortable truth: we’re still learning how to interpret viral signals in the food chain.
Testing results are a signal, not a control. The more proactively we use that signal, the better. Get trending. Be a trend-setter.
Until testing becomes more harmonised, transparent, and predictive of real-world health risks, we’re left acting on the best available evidence – but never quite the whole picture.
[1] Viral Contamination of Food – PMC
[2] Frozen corn likely source of ongoing Listeria monocytogenes outbreak | EFSA
[3] Virus testing UKAS accreditation at Campden BRI
[4] FDA Sampling Frozen Berries for Harmful Viruses | FDA
[5] RASFF – European Commission
[6] Moroccan strawberries cleared of Hepatitis A contamination | Flowers & Fresh Produce Middle East & Africa Magazine
