A Rare Newborn Infection That Is Slowly Becoming Less Rare

What the first worldwide study of neonatal herpes found, why the infection is so easily missed, and what it means for families

Neonatal herpes is an uncommon but serious infection that a baby can catch around the time of birth. The first worldwide study of how often it happens estimates that it affects roughly one baby in every 10,000 born, and that this number has been creeping upward by about 3.5% each year. This article explains what researchers found, why the infection is so difficult to recognise in a newborn, and what it means for families and their baby's care.

What this infection is, and why it hides so well

The worldwide study behind that one-in-10,000 figure counted real, laboratory-confirmed cases rather than estimating them from models [1] — but before the number can mean much, it helps to understand what the infection actually is and why it is so easy to miss.

Herpes simplex virus is extremely common in adults. Most people who carry it have either cold sores around the mouth or genital herpes, and many have no symptoms at all and never know they are infected. In adults it is usually a nuisance rather than a danger. In a newborn, whose immune system has not yet learned to control viruses, it can be devastating.

About 85% of newborn infections are passed on during birth, when the baby comes into contact with the virus in the birth canal [2]. The risk is highest not for women who have known about their herpes for years, but for women who catch it for the first time late in pregnancy. A long-standing infection gives a mother time to make protective antibodies that cross the placenta and shield the baby; a brand-new infection does not [3]. This is the cruel arithmetic at the heart of the problem: the mothers whose babies are most at risk are often the ones who have no idea they are carrying the virus, and therefore no history to mention to anyone.

Doctors sort newborn herpes into three patterns. The mildest, called SEM disease, affects the skin, eyes and mouth and produces small blisters. A second form attacks the brain and is called CNS, or central nervous system, disease. The third and most dangerous, called disseminated disease, spreads through the bloodstream to the liver, lungs and other organs [4]. Treatment is an antiviral medicine called aciclovir, given into a vein, and it works — but how well it works depends heavily on how quickly it is started.

How families and doctors used to face this problem

Before effective antiviral drugs existed, a diagnosis of newborn herpes was close to a death sentence for babies with the brain or bloodstream forms. Even after treatment became available, the picture in the UK was bleak. The first national count of these babies, gathered between 1986 and 1991 through a system called the British Paediatric Surveillance Unit — a network in which paediatricians across the UK and Ireland report rare conditions each month — found about 1.65 cases per 100,000 births, and more than half of the 76 babies reported either died or were left with significant disability [5].

When the same network repeated the exercise for babies born between 2004 and 2006, it found 85 confirmed babies, or 3.58 cases per 100,000 births — roughly double the earlier figure. Twenty-six per cent of those babies died. And crucially, more than 70% had the brain or bloodstream form of the disease, and of those, over half had no skin blisters at all [5]. The single sign most people associate with herpes was simply absent in the babies who were sickest.

For a long time, this was almost all anyone could say with confidence, and only for a handful of wealthy countries. A national study in the United States estimated about 10 cases per 100,000 births in 2006 [6]. In 2017, researchers produced a first global estimate — around 10.3 cases per 100,000 births — but it was a calculation built from adult infection rates rather than a count of actual affected babies [7]. Nobody had ever gathered together all the real-world counts that did exist and asked what they added up to.

What the new global study did

That is what a research team based at Weill Cornell Medicine–Qatar in Doha, working with colleagues in New York and Qatar, set out to do [1]. They searched two of the world's largest medical research databases from their earliest records through December 2024, and added national reporting programmes from Australia, Britain, Canada, Germany, Ireland and Switzerland. From more than 13,000 records they identified 143 that met strict quality standards — cases had to be confirmed by laboratory testing of more than one part of the body, not guessed from symptoms or from a single test.

Pulling those 143 reports together gave a worldwide estimate of 8.2 cases per 100,000 live births, or about one baby in every 10,000. But that global average hides big differences. In the Americas the rate was 13.3 per 100,000; in Europe, 5.2; in the Western Pacific region, 2.9. The team also found that the rate has been rising by about 3.5% a year, and that the type of virus responsible is shifting: the mouth-associated type (HSV-1) is becoming more common as a cause of newborn infection, gaining roughly 1.4% each year, while the genital type (HSV-2) is slowly giving ground.

The researchers are careful about what this does and does not mean. Their data came from only three of the world's six regions — nothing at all from Africa, the Middle East or South-East Asia, where the infection is hardest to diagnose. The countries they could include account for just 6.1% of the world's births. Their own view is that 8.2 per 100,000 is probably an undercount rather than an overcount.

What recent records from real hospitals show

Two studies published around the same time put faces to those numbers. The British surveillance network ran a third count, covering August 2019 to February 2022, and found 117 babies — a rate of 6.0 per 100,000 births, again about double the count from fifteen years earlier [8]. And in California, a review of 632,979 babies cared for by a large health system over 16 years found 62 cases, or 9.8 per 100,000 births, with premature babies affected roughly three times as often as babies born at term [9].

The British study is the one that should reshape how hospitals think. Babies typically became unwell around day 8 of life. Among the babies with the bloodstream form, nearly two-thirds had no fever and almost three-quarters had no blisters. Their blood tests for inflammation, which usually rise sharply in a bacterial infection, were normal. More than 80% were born to mothers with no known history of genital herpes. Over half of the babies did not start antiviral treatment until more than 24 hours after they first became unwell. Just under a quarter of all the babies died, and among those with the bloodstream form the figure was much higher. Of the 41 babies followed up at two years of age, about three in ten had some degree of developmental difficulty [8]. Outcomes have not improved much since the previous count, and similar results have been reported from North America [10].

Why the numbers may be climbing

The explanation researchers favour has to do with changes in the adult population rather than anything about newborn care. Fewer children now catch the cold-sore virus in early childhood, which means more people reach adulthood without immunity and encounter the virus for the first time through sexual contact — so genital infections caused by the cold-sore type have been rising by 1–4% a year in the countries with data [11], [12], [13]. There is also a subtler effect: as the older genital type becomes less common overall, people who do catch it tend to catch it later in life, which pushes more first-time infections into exactly the years when people are having babies [1]. Neither trend has anything to do with parental behaviour or with anything a family could have done differently.

What this means for you and your baby

If your baby is being tested for herpes or started on aciclovir, it is usually a precaution, not a diagnosis. Because the treatment works best when it is started before anyone can be certain, doctors deliberately treat many babies who turn out not to have the infection. Being tested is a sign that the team is taking a small but real possibility seriously — it is not a sign that they think your baby has herpes.

If you have no history of herpes, that does not mean anything was overlooked during your pregnancy. Most mothers of affected babies have no history either, because their infection was new and silent. Nothing about this is a failure of care or of parenting.

What families can reasonably ask for is attentiveness. If your newborn becomes unusually sleepy, feeds poorly, has a seizure, or simply seems wrong to you in the first three weeks of life, say so plainly and ask whether herpes has been considered — particularly if there is no fever and no rash, because those are precisely the babies who get missed. Trusting your own sense that something has changed is not overreacting.

What researchers are working on next

Three things are underway. The first is making newborn herpes a formally reportable condition in more countries, so that trends can be tracked rather than reconstructed years later. The second is finding out whether the shift toward the cold-sore type of virus is changing how severe the illness tends to be — an open question that will need studies designed specifically to answer it, since the genital type has historically been linked to the more serious forms [10]. The third, and the one with the greatest potential, is a vaccine given to adults that would prevent the maternal infections behind almost every newborn case [14]. Researchers are also beginning to understand why a small number of babies become unusually severely ill, which appears in some cases to involve inherited differences in how their immune systems handle viruses [15]. None of that will change what happens on a ward this week. What can change this week is how quickly an unwell newborn is offered treatment — and that is the finding worth carrying away.

References

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