COVID-19 may have faded from news headlines, but it hasn’t gone away – or stopped evolving. The World Health Organization (WHO) currently lists six ‘variants under monitoring’ — meaning they may require prioritised attention and tracking by health authorities due to their increased prevalence and potential public health implications.

One of them is NB.1.8.1, unofficially nicknamed “Nimbus” by the group of researchers and citizen scientists who’ve proposed previous monikers like Kraken, Centaurus and Eris for variants they thought people would be talking about.

Another variant raising concern among virus spotters – though not currently designated for monitoring by the WHO — is XFG, informally nicknamed “Stratus”.

Here’s what we know about these two variants so far…

1. They’re spreading fast – but aren’t necessarily any nastier

Since being identified in January 2025, NB.1.8.1 has quickly spread across Asia and into other regions, accounting for 11% of sequenced cases by late April 2025 – up from 2.5% one month earlier.

This is rapid compared to other co-circulating variants, but while increases in cases and hospitalisations have been reported in some countries – particularly in the Western Pacific Region — there are no reports to suggest that NB.1.8.1 is associated with more severe disease compared to other circulating variants.

In its latest risk evaluation for NB.1.8.1 published on 23 May 2025, WHO’s Technical Advisory Group on Virus Evolution said: “The available evidence on NB.1.8.1 does not suggest additional public health risks relative to the other currently circulating Omicron descendent lineages.”

XFG was first detected in Canada and has also achieved rapid global spread. In Europe, it accounted for 25% of cases as of late May, while NB.1.8.1 cases stood at 9%. It is also taking off in India, where 206 cases had been detected as of June 11.

In its risk evaluation for NB.1.8.1, WHO noted that XFG was spreading even faster than this variant. However, there’s similarly no evidence to suggest it is associated with more severe disease at the current time.

2. Nimbus isn’t a Harry Potter reference

According to Prof T. Ryan Gregory at the University of Guelph in Canada – who is part of a group of researchers and citizen scientists that assigns unofficial nicknames for COVID variants likely to attract public attention — NB.1.8.1 or “Nimbus” is the first variant to receive such a nickname since “Pirola” (BA.2.86) in August 2023.

“It’s largely been the Pirola show since then, with nearly everything in circulation being a direct descendant of it, or a recombination of its descendants,” Gregory told VaccinesWork.

“Several factors have gone into giving NB.1.8.1 the first nickname in nearly two years: It is not just a member of the Pirola ‘variant soup’, it has established itself as a new lineage that is now evolving among hosts, and it has caused waves in some parts of the world — and has potential to do so more broadly. It was also listed as a variant under monitoring (VUM) by the WHO as of May 23.”

While some people might assume ‘Nimbus’ to be a reference to the Nimbus 2000 broomsticks used by Harry Potter, the Nimbus spacecraft in the Futurama TV series, or even the villainous Mr Nimbus the Rick and Morty show, Gregory said the name refers to a type of cloud. “Plus it’s helpful that it contains ‘n’ and ‘b’,” he said.

He added that the decision to also assign the XFG variant a nickname was born from the observation that “Nimbus (NB.1.8.1) and Stratus (XFG) are the two main variants competing for dominance globally and either one has the potential to kick off a surge.

“Generally, we’re very conservative about giving new nicknames, and then we bounce suggestions back and forth, trying to find something that is recognisable enough to be useful in broad communications,” Gregory said.

“We started with mythological creatures (e.g., Orthrus, Kraken), then astronomical names (e.g., Eris, Pirola), and this time around we’ve used some terms from meteorology including the cloud types Nimbus and Stratus.”

3. These are Frankenstein variants, built from genetic recombination

NB.1.8.1 has a tangled family tree. It is a hybrid virus, built from a process called recombination, where several viruses – all versions of the Omicron COVID variant -infect the same cell and swap genetic material. It is descended from XDV — a hybrid of JN.1 and XDE (a hybrid of GW.5.1 and FL.13.4).

Credit: T Ryan Gregory.

These events have equipped “Nimbus” with seven new mutations in its spike protein, which is the part of the virus that attaches to human cells, plus a further 20 changes elsewhere.

XFG is a mix of two earlier variants: LF.7 and LP.8.1.2. It has four key mutations in its spike protein.

Credit: T Ryan Gregory.

4. Nimbus sticks to our cells and infects them really well

Virological studies suggest that NB.1.8.1 binds strongly to the ACE2 receptor that SARS-CoV-2 uses to enter human cells – more strongly than XFG and most other variants.

It also shows strong infectivity in lab-grown cells, suggesting it’s well-equipped to spread. Its infectivity (may also be 2.5 times higher than that of LP.8.1 — currently the most prevalent COVID-19 variant worldwide.

This could help explain why it is muscling out other variants – not because it evades the immune system better, but because it infects people more efficiently.

5. Stratus is good at evading the immune system

Two of XFG’s spike mutations are thought to help it evade antibodies that target common spots on the spike protein. One of these may also help it to dodge a broader class of antibodies, making it harder for the immune system to neutralise, and easier for the virus to spread.

However, XFG doesn’t bind to the human ACE2 receptor as well as some other variants, so it may not be as good as infecting cells or spreading on its own – unless it picks up additional mutations.

6. COVID-19 vaccines are still likely to be effective against them

In lab tests using blood samples from vaccinated and previously infected individuals, XFG showed a roughly 2-fold drop in antibody effectiveness compared to the current most common COVID‑19 variant globally, while NB.1.8.1 showed a roughly 1.6-fold reduction.

Even so, people who are fully vaccinated – especially those with updated boosters – are still likely to have some protection, especially against severe illness, hospitalisation and death.

7. There may be some specific symptoms to watch out for

While reports suggest that symptoms of these variants are broadly similar to those of other versions of Omicron, UK doctors have warned that a “razor blade” sensation in the throat and gastrointestinal symptoms such as nausea, vomiting, diarrhoea, heartburn, bloating, constipation, and stomach pain, could be particular signs to look out for with NB.1.8.1 – although this hasn’t been confirmed in scientific studies.

Indian doctors have also flagged a hoarse voice as common among patients who’ve recently presented at hospital outpatient departments there.

8. Nimbus is under close watch, but is not a top tier worry

WHO has designated NB.1.8.1 as a Variant Under Monitoring (VUM), not a Variant of Interest or Concern. That means it’s noteworthy, but not alarming — at least for now. Its overall public health risk is currently assessed as “low”.

XFG is not currently deemed a threat worthy of enhanced WHO monitoring. However, like all emerging variants, it remains under routine genomic surveillance, and all uploaded virus sequences are being closely tracked for changes in spread, mutations, or potential public health impact.

As the United States faces its largest measles outbreak in years, parents and doctors alike are having to educate themselves about the virus, which was declared eliminated in the country 25 years ago.

“I’ve been doing this for 20 years, and up until 2019, I had never seen [a case of] measles,” says Aaron Milstone, MD, MHS, a pediatric infectious disease specialist at Johns Hopkins Children’s Center and a professor of pediatrics at the Johns Hopkins University School of Medicine in Baltimore.

The last outbreak, in 2019, occurred in communities with low vaccination rates in New York City; that year, the country had a total of 1,274 reported cases, according to data from the Centers for Disease Control and Prevention (CDC). As of April 24, 2025, there have already been 884 reported cases and three deaths this year, mostly tied to an outbreak in West Texas among unvaccinated individuals.

“We’re on track to eclipse [the 2019 outbreak,] for sure,” says Milstone, noting that the official numbers are likely underestimates, since many people don’t report mild cases. This outbreak is also more widespread, with cases reported in more than half of U.S. states.

“We need to be prepared, as more and more states are reporting cases,” says Scott Roberts, MD, associate medical director of infection prevention at Yale New Haven Health and an assistant professor at the Yale School of Medicine. “It does need to be on clinicians’ radars.”

While most people in the United States are protected from the virus through vaccination or previous infection, measles is one of the most contagious viruses in existence, so health care providers should familiarize themselves with the symptoms, and hospital leaders should have a plan for handling a local outbreak, Milstone says.

Before the measles vaccine was introduced in 1963, each year, approximately 3 million to 4 million people in the United States were infected, 400 to 500 died, 48,000 were hospitalized, and 1,000 were diagnosed with encephalitis, a swelling of the brain that can cause serious complications, including brain damage, hearing loss, intellectual disabilities, and death.

Still, despite the measles vaccine being 97% effective at preventing infection, increasing vaccine hesitancy and misinformation about vaccine safety have resulted in a decline in parents getting their children vaccinated. This has opened up the opportunity for outbreaks.

AAMCNews interviewed infectious disease experts from academic medical centers to help answer common questions about the measles virus, the vaccine, and how health care providers can help prevent another epidemic.

How does measles spread, and what are the symptoms?

Measles is one of the most contagious viruses in existence and will infect 9 out of 10 people who come into contact with an infected person if those not yet infected don’t already have immunity, according to the CDC. It spreads through airborne particles from an infected person’s cough or sneeze. These particles can linger in a poorly ventilated indoor space for up to two hours after the person has left the room.

Public health infection control protocols are similar to those implemented during the COVID-19 pandemic, including encouraging handwashing, wearing masks, disinfecting high-touch surfaces, and requiring people who test positive for the virus to stay home and avoid others who don’t have prior immunity or are immunocompromised.

All U.S. states and the District of Columbia require measles vaccination for children attending school, but exemptions to the mandates have been inching up in recent years.

Measles symptoms generally appear seven to 14 days after contact with the virus and can initially include fever, cough, runny nose, diarrhea, vomiting, and watery eyes. After two to three days of symptoms, tiny white spots might appear inside the mouth, and after three to five days, the telltale measles rash (flat red spots) begins to appear, starting on the face and spreading down the body. A person with measles can be contagious for up to four days before and four days after the rash appears.

“From a health care worker perspective, the problem with measles is severalfold,” Roberts says. “The disease mimics other things. A patient may present with fever and runny nose, but the rash comes on later. I have three kids in preschool, and my kids meet those criteria every day. It’s tough” to catch measles early.

Measles can have rare, but serious complications, including blindness, pneumonia, encephalitis, and death.

What should I do if I think I or my child might have measles? Are there any treatments?

People exhibiting measles symptoms and planning to seek medical help should call ahead to their doctor’s office or hospital to inform them of a possible measles case before arriving, Milstone says.

He emphasizes that this is essential because pediatrician offices and hospital emergency departments are often visited by people who are particularly vulnerable to the severe complications of measles. These include newborns and babies who are too young to be vaccinated, unvaccinated pregnant women and their unborn babies, and immunocompromised people.

Roberts says that, additionally, putting a mask on the sick person can also help prevent spreading the infection.

“One thing we’ve learned [from the COVID-19 pandemic] is that masking can reduce risk of transmission of almost any pathogen.”

While there is no approved treatment for measles, doctors can offer some help with symptom relief. The World Health Organization recommends fluids to help with dehydration from diarrhea and vomiting, antibiotics to treat pneumonia and ear infections that arise due to the virus, and vitamin A supplements to help prevent blindness.

The American Academy of Pediatrics (AAP), however, warns that claims that vitamin A can prevent measles are false.

“Vitamin A is recommended for children diagnosed with measles to help prevent complications, particularly in children who are hospitalized,” said Sean T. O’Leary, MD, MPH, chair of the AAP Committee on Infectious Diseases in a press release on Feb. 27, 2025. “It should not be used to try to prevent measles, and high doses of vitamin A are potentially very harmful. The only effective way to prevent measles is the MMR [measles, mumps, and rubella] vaccine.”

Who should be vaccinated? Should I get a booster?

The MMR vaccine, which is administered in two doses to children, one at 12 to 15 months old and another at 4 to 6 years old, is 97% effective at preventing infection. A single dose of the vaccine is 93% effective against infection. According to the CDC, a community must have 95% immunity from either the vaccine or prior infection to prevent an outbreak. Gaines County, the epicenter of the West Texas outbreak, had a vaccination rate of 82% among kindergartners last year, state records show.

Unlike with COVID-19 and the flu, the measles virus does not mutate enough to require additional vaccinations, so most people who have been vaccinated or who have had a prior infection will have robust immunity for life, Roberts explains. This also means people who were born before 1957 and never got vaccinated (the vaccine became available in 1963) are still protected, because experts believe all people born in that period were infected at some point and have natural immunity.

Any eligible child or adult who is unvaccinated or does not have documentation of their vaccination or a confirmed prior infection should speak with their health care provider about getting the shot.

Adults who were vaccinated between 1963 and 1967 should also speak with their health care provider about a blood test for immunity or a vaccine, since early vaccines were not as effective as those available today. The CDC says there’s no harm in getting an additional vaccine if immune status is unknown.

Babies can receive a first dose of the vaccine as early as 6 months old if they live in an area with an active outbreak or will be traveling internationally, according to the CDC.

Before 1989, most children received only one dose of the vaccine, so anyone in that group who is in a high-risk situation, such as health care workers, might consider getting a second dose for additional protection, Roberts adds.

However, since the vaccine does include a live virus, the CDC recommends against vaccination for people who have weakened immune systems — for example, those on immunosuppressant drugs for organ transplants, and pregnant women.

The CDC also says that unvaccinated people with a known exposure should seek care immediately. They can receive either a dose of the MMR vaccine within 72 hours of exposure or an infusion of antibodies within six days, though not both, to help prevent serious disease.

What can health care providers do?

In the United States, 91% of children age 2 and older had gotten one dose of the MMR vaccine as of 2021, according to CDC data, and as of the 2023-2024 school year, 93% of kindergartners had been vaccinated.

But these numbers may not provide an accurate picture, since they don’t include all children under age 5 for the past four years.

That’s why Eric G. Zhou, PhD, an instructor of pediatrics, cardiology, and population health science and policy at the Icahn School of Medicine at Mount Sinai in New York City, and a team of researchers have used digital health surveys to gather information on measles vaccination trends.

One study, published in March 2025, which asked 20,000 parents of children under 5 about their children’s measles vaccination status, found that the vaccination rate for this age group could be as low as 71% — lower than CDC statistics suggest and below the 95% needed for herd immunity.

Though the study had limitations, including self-reporting bias and the fact that some parents may have reported vaccination status for babies not yet eligible for the vaccine, it raises concerns that more young children could be susceptible to measles than national data suggests, Zhou says.

“The general U.S population experienced a tremendous disruption from the COVID-19 pandemic in seeking care,” he says. “We see this evolving parental hesitancy, which shows the importance of establishing trust and transparency in developing vaccines and using data and evidence to alleviate some of these concerns.”

Physicians may have a significant role in assuaging people’s doubts regarding vaccines. Some studies have shown that patients with doubts about a vaccine are more likely to accept it if their physician recommends it directly and assertively. The AAP offers guidance for pediatricians on how to effectively reassure parents regarding vaccination.

Additionally, targeting public health outreach to communities with low vaccination rates could help.

One study found that, during the 2019 measles outbreak in several communities in New York City, vaccination promotion in these areas increased the vaccination rate from 80% to 91%.

But with proposed budget cuts to national health care agencies, Milstone worries public health efforts could be stymied, putting even more responsibility on leaders in academic medicine to speak up.

“The public health outreach is going to be critical,” he says. “Academic health centers should play an active role in public health messaging.”

Whether you’re welcoming home a new baby, flying to a new destination for a vacation or working around sick patients as a doctor or nurse, your immunity is your invisible armor against all the bugs and diseases lurking out there.

Checking to see how strong that armor really is under these and other circumstances is a priority to individuals, educators and employers. Titer tests are how such strength can be measured. These quick and easy blood tests reveal whether your body has enough antibodies to fend off the infections you’re hoping to keep away and can also tell you if a booster is in order.

Here’s what a titer test is, how one is administered and what results you can expect from taking one.

What is a titer test?

A titer test is a simple blood test that identifies whether your body has any antibodies associated with a specific disease. An antibody is a protein that’s created by your body’s immune system to fight bacteria and viruses. “Prior to getting a vaccine or being exposed to a viral infection, our bodies have little to no antibodies against that specific vaccine or virus,” explains Cynthia Leifer, a professor of microbiology and immunology at Cornell University. But once our body has already been exposed to a virus or vaccine, “our antibody levels against it increase dramatically,” she says.

Titer tests are what’s used “to help you determine if you currently have immunity to certain diseases due to past infection or vaccination,” says Dr. Alison Mitzner, a New York board-certified pediatrician and the author of “Calm and Confident Parenting.”

In other words, whether you’re going to be exposed to varicella (chickenpox), hepatitis, rubella, COVID-19 or the measles or mumps, these tests will help you know if you’ll be protected. And for some of these diseases that provide long-term or lifelong immunity after exposure, a single titer is all that is needed to know if protection is still intact. But for viruses like COVID-19, multiple tests may be necessary as such antibody levels ebb over time.

How is titer testing given?

A titer test is administered through a standard blood draw that’s both quick and usually painless. No special preparation is necessary, and the procedure takes only a few minutes. “They are administered by a healthcare provider at the doctor’s office, in the hospital, at various pharmacies or at laboratory diagnostic centers,” says Mitzner.

A few days after your blood has been drawn, your healthcare provider will receive the results and contact you “to go over the results and to make recommendations for any treatments or vaccines,” says Leifer.

Did you see? What experts need you to know about the MMR vaccine

Who is titer testing for?

Just about anyone can get a titer test. They’re especially valuable for people who don’t have a complete record of their past vaccinations, pregnant women and individuals who are frequently interacting with large groups of people. Healthcare professionals and healthcare students also usually want them because of the number of sick patients they are frequently exposed to.

Parents of a new baby, people with autoimmune disorders or symptoms “and individuals who may have been exposed to an infection while traveling abroad may want clear evidence of immunity against specific diseases or need a titer test to confirm their immune response,” says Mitzner.

Sometimes employers and school systems also recommend for an employee or a student to take a titer test, particularly if that employee or student doesn’t have a complete record of past vaccinations.

What is Tdap? Here’s what you need to know to stay protected against whooping cough

What do titer test results mean?

Titer tests can show if you have active immunity that you’ve received directly from a past infection or vaccine; passive immunity that’s been passed from mother to baby or from donated blood; or no immunity at all.

And titer tests don’t just determine if you have immunity, they can also help you determine just how protected you really are; they quantify the number of specific antibodies in your blood. “Titer tests measure both the amount and type of antibodies in a blood sample,” explains Leifer. “This information is used to help you determine if you should receive treatment for a viral infection you may have been exposed to or be given a vaccine or booster to protect you against one going forward.”

3. Robust, practical evidence for vaccine impact on AMR is urgently needed

While modelling studies have provided valuable estimates of the potential impact of vaccines on AMR, there is a pressing need for more empirical, real-world data, especially from low- and middle-income countries (LMICs) where the burden of AMR is often highest.

The Wellcome-funded projects reviewed in the report encompass a wide range of settings, pathogens, and methodologies, revealing significant complexity in the relationship between vaccination, antimicrobial use, and AMR. For instance, studies on influenza, typhoid, pneumococcal, malaria, and diarrhoeal diseases show that vaccines can reduce antibiotic prescribing, but the effects are inconsistent and influenced by local health system factors, diagnostic practices, and patient behaviours.

Currently, studies use a wide variety of metrics to assess impacts on antimicrobial use and AMR genes, making it difficult to compare results or conduct meta-analyses.

A major barrier to synthesising evidence on the impact of vaccines on AMR is the lack of standardisation in outcome measures, study designs, and analytical approaches.

Furthermore, even when reductions in antibiotic use or AMR gene prevalence are observed, the biological and clinical significance of these changes is not always clear. The report underscores the importance of embedding AMR endpoints in vaccine trials and observational studies, and of strengthening surveillance systems to generate robust, context-specific evidence that can inform policy and practice.

The Wellcome report calls for greater alignment with emerging frameworks, such as the WHO’s considerations for research studies evaluating vaccine impact on AMR, which recommend standardised protocols and priority outcome measures. Standardisation would facilitate the aggregation of data across studies, enable more robust policy guidance and help identify where vaccines can have the greatest impact on AMR.

The report also highlights the need for international collaboration to develop and implement these standards, as well as for investments in surveillance infrastructure, particularly in resource-limited settings. By adopting common metrics and methodologies, the global health community can accelerate the generation of actionable evidence and support more effective decision-making on vaccine use for AMR control.

5. Evidence generation should prioritise pathogens and/or settings where it can most influence policy

Not all vaccines or pathogens offer the same potential for AMR impact, and resources for research and implementation are limited. The report recommends prioritising evidence generation for pathogens and contexts where AMR outcomes could be most influential in policy decisions. By focusing on high-impact scenarios and integrating AMR considerations into vaccine investment strategies, policymakers can make more informed decisions that maximise public health benefits and help curb the spread of resistance.

This includes situations where the disease burden or vaccine efficacy is borderline, and strong evidence of AMR benefits could tip the balance in favour of vaccine introduction.

It is particularly relevant for pathogens with limited treatment options due to resistance, such as gonorrhoea or Shigella, or where vaccines could significantly reduce antibiotic use.

The report also stresses the importance of aligning research with policymaker needs, including economic analyses and syndromic data, to ensure that evidence is both relevant and actionable.

By focusing on high-impact scenarios and integrating AMR considerations into vaccine investment strategies, policymakers can make more informed decisions that maximise public health benefits and help curb the spread of resistance.

6. Policy decisions require evidence that is aligned with real-world clinical management

For evidence on vaccines and AMR to influence policy, it must be presented in ways that reflect how antibiotics are actually prescribed and how clinical care is delivered. In many settings, especially where diagnostic capacity is limited, antibiotics are prescribed based on syndromic presentation (such as fever or diarrhoea) rather than confirmed pathogen diagnosis. The report suggests that presenting AMR data within the context of clinical syndromes, rather than only for individual pathogens, can better inform decision-making and align with existing clinical guidelines. This approach acknowledges the realities of healthcare delivery and the challenges of implementing pathogen-specific interventions in resource-limited environments.

JOHANNESBURG – How do you know when and if you should vaccinate your child?

The debates around vaccinations have gone on for many years, with different views on the matter and since the Covid-19 pandemic many opinions have been changed or reshaped.

Vaccine hesitancy is still prominent but as a parent or even as a person without children, what should we know when it comes to vaccinations?

Dr Sarah Cooper specialist scientist at Cochrane South Africa, spoke to eNCA.