The resurgence of vaccine preventable infections: Measles and Pertussis

Over the last decade, there has been a concerning decline in childhood vaccination rates in the UK. This has been mirrored in many parts of the world. This trend, exacerbated by the COVID-19 pandemic, has led to a resurgence of vaccine-preventable diseases such as measles and pertussis. Once on the brink of eradication, these diseases now pose significant public health risks, especially to vulnerable groups like infants, young children, and the immunocompromised.

This article will explore the factors behind the increase in measles and pertussis cases, healthcare professionals’ role in reversing these trends, and strategies to help reduce spread.

A Decline in Vaccination Coverage: A Global Issue

In 2023, the World Health Organisation (WHO) reported that worldwide immunization rates for key vaccines, including the diphtheria-tetanus-pertussis (DTP3) series, had dropped to 84% from pre-pandemic rates of 86%. Similarly, the global vaccine coverage of children who received a first dose of measles vaccine was 83% in 2023, compared to pre-pandemic levels of 86% in 2019.

In the UK, vaccine coverage for measles and whooping cough is below the World Health Organization’s recommended 95%. This is the benchmark required to effectively halt the transmission of these highly contagious diseases.

In the UK, the latest vaccination coverage rates from 2023-2024 show that only 84.3% of children have received both doses of the measles-mumps-rubella (MMR) vaccine, while just 82.8% have completed their preschool DTaP/IPV booster. These figures fall concerningly short of the herd immunity threshold, leaving the population vulnerable to outbreaks. The reduced coverage is particularly notable in urban areas like London, where socio-economic and cultural barriers further compound the problem. This contributed to a sharp increase in cases of both diseases in 2024. A review of child deaths in England and Wales from 2019 to 2022 revealed that delayed or missed vaccinations were a contributing factor in 3% of cases.

Measles: An Early Warning System

Measles acts as an early warning system in countries with vaccine uptake below the recommended threshold of 95% required to achieve herd immunity. It is one of the most transmissible diseases and spreads quickly from person to person, mainly via the respiratory route. Young infants and immunosuppressed individuals are particularly at risk of severe disease, resulting in significant morbidity and associated mortality rates of 1-3 per 1,000 cases.

Measle symptoms commence with a prodromal phase lasting 2-4 days before the rash develops. Symptoms include high-grade fevers, coryzal symptoms, cough and conjunctivitis – which may differentiate the illness from other influenza-like illnesses.

The characteristic maculopapular rash starts on the face and behind the ears before spreading down the trunk and extremities, and its appearance often coincides with a peak of symptoms. Koplik spots in the mouth may also appear at this time. Measles can lead to several complications, including viral pneumonitis, otitis media, secondary bacterial pneumonia, and encephalitis (occurring in 1 in 1,000 cases). In rare instances, it may also cause subacute sclerosing panencephalitis (SSPE), a serious and often fatal condition affecting approximately 1 in 10,000 cases.

Since January 2024, measles cases in the UK have surged, reaching 2,563 reported cases—the highest level since 2012. Most cases have occurred in children under 10, with a significant concentration in London.

Countries with higher measles vaccination coverage, like Australia—where 93% of children received two doses of the MMR vaccine in 2023—continue to report low numbers of measles cases. Most cases in these regions occur in unvaccinated individuals returning from overseas. Since 2022, measles cases have risen steadily worldwide, with significant outbreaks reported in several South Asian and African countries. These outbreaks increase the risk of imported cases in other regions, including the UK.

Whooping Cough: A Cyclical but Preventable Disease

Whooping cough, caused by the bacterium Bordetella pertussis, is another infectious disease experiencing a resurgence despite the availability of vaccines. Unlike measles, pertussis follows a cyclical pattern, with incidence peaking every 3–5 years. This cyclical pattern is partly driven by waning immunity, even within vaccinated populations. Unlike the measles vaccine, which provides lifelong immunity, the pertussis vaccine’s protection diminishes over time, making booster doses essential to sustain immunity.

Pertussis typically begins with cold-like symptoms, including a cough, low-grade fever, and nasal congestion (known as the catarrhal stage). This progresses to the paroxysmal stage, marked by intense coughing fits followed by the distinctive “whoop” sound as the patient breathes in. In infants, however, this whooping sound may be absent, and they are at risk of apnoea. Pertussis is especially dangerous for infants, carrying a higher risk of severe illness and mortality, with approximately 1% of cases resulting in death among those under two months old. In older children and adults, pertussis often presents as prolonged coughing spells that can persist for weeks and may lead to complications such as pneumonia.

In the UK, pertussis cases have steadily increased throughout 2023, with a significant resurgence in 2024. Between January and August 2024, over 13,000 laboratory-confirmed cases of pertussis were reported to the UK Health Security Agency, with tragically ten infant deaths attributed to the infection.

The disease tends to be underdiagnosed in adolescents and adults. They often experience milder symptoms but can still transmit the infection to vulnerable populations, including infants and the elderly and contribute to sustained transmission.

Following a 2012 outbreak, an emergency maternal pertussis vaccination programme was introduced in the UK, offering the vaccine to women in the third trimester of pregnancy. This strategy provides passive immunity to infants through the transfer of maternal antibodies, protecting them from birth until they receive their first pertussis vaccination at 8 weeks as part of the routine schedule. In 2019, the Joint Committee on Vaccination and Immunisation (JCVI) recommended that the antenatal pertussis vaccine become part of a routine programme. Pregnant women are now advised to receive a pertussis booster with each pregnancy. However, awareness and uptake of this recommendation remain low, and vaccine uptake rates have declined in recent years.

Why are rates so high in the UK?

Vaccination uptake

The most effective strategy to reduce measles cases is to achieve high coverage of both doses of the measles, mumps, and rubella (MMR) vaccine within the population. The MMR vaccine offers 93% effectiveness after the first dose and rises to 97% after the second dose.

Achieving high and sustained vaccine coverage is essential to eliminating endemic measles. London remains the most vulnerable region, with many birth cohorts—especially younger children in primary and secondary schools—falling short of immunity targets. Vaccine uptake also varies considerably by ethnicity, socioeconomic status, and location, leaving under-vaccinated communities disproportionately affected by measles outbreaks.

Maternal vaccination with the pertussis vaccine (Tdap) between 20 and 32 weeks of pregnancy is highly effective, reducing the risk of infant death from pertussis by 92%.

Despite the clear benefits, pertussis vaccination coverage during pregnancy has declined from 75% in 2016 to just 58.6% in 2023–2024. Like the MMR vaccine, pertussis vaccine uptake is lowest in London, with some areas reporting coverage as low as 24%.

COVID-19

The COVID-19 pandemic has led to a global decline in routine childhood immunisation rates and widening immunity gaps. Health services worldwide were disrupted, with routine immunisation programmes among the hardest hit. Many children missed scheduled vaccinations due to lockdowns, healthcare facility closures, or concerns about contracting COVID-19 in medical settings. Although efforts have been made to catch up on missed vaccinations, substantial gaps persist, particularly in resource-limited settings.

Globally, health systems continue to struggle with the after-effects of the pandemic, delaying efforts to catch up on missed vaccinations. In high-income countries like the UK, the main challenge is rebuilding public trust and encouraging parents to ensure their children receive missed doses. In low- and middle-income countries, however, these efforts are further complicated by infrastructure limitations and competing health priorities.

Mode of transmission and spread

Measles spreads through respiratory droplets when an infected person coughs, allowing the virus to cover a wide area rather than being confined to direct contact. It can linger in the air or on surfaces for up to two hours, making it highly contagious and capable of infecting others even after the infected person has left the room. The reproductive number (R0) for measles is between 12 and 18, meaning each infected individual can, on average, transmit the virus to 12–18 unvaccinated people.

Additionally, individuals with measles are infectious for up to four days before the appearance of the characteristic rash, allowing the virus to spread even before a case is recognised and isolated.

Pertussis spreads through respiratory droplets and has a high reproductive number (R0) of 15–17, meaning each infected individual can potentially transmit the infection to 15–17 others. The extended infectious period—lasting up to 21 days after the onset of cough—further contributes to its high transmission rate. After receiving chemoprophylaxis, contacts may remain infectious for up to 48 hours.

A cross-sectional surveillance study conducted in the Netherlands revealed that pertussis rates in the population were significantly higher than previously estimated, with an observed prevalence of 5.9% compared to the earlier estimate of 3.5%.

The most notable increase in pertussis prevalence was observed among 12–18-year-olds who had received the whole-cell pertussis vaccine as a primary series and an acellular booster. Additionally, a systematic review in 2020 found a high prevalence of subclinical pertussis infections among household contacts of confirmed cases. These asymptomatic or mild infections may significantly contribute to the ongoing transmission of pertussis and lead to an underestimation of its true incidence in the population.

Diagnostic challenges

A key challenge in diagnosing measles is its similarity to several other, more common rash-causing illnesses, which can lead to misdiagnosis. Conditions such as roseola, scarlet fever, rubella, glandular fever, and fifth disease (caused by parvovirus B19) often present with similar symptoms, complicating the clinical diagnosis of measles. This leads to delay or misdiagnosis in the ED, which may lead to missed measles cases.

Similarly, whooping cough symptoms can be mistaken for those of other respiratory viruses, leading to delayed diagnosis. The clinical case definition for pertussis is broadly consistent worldwide, with key features including paroxysmal coughing, post-tussive vomiting, inspiratory whoop, apnoea, and a cough lasting at least two weeks.

To meet CDC and WHO diagnostic criteria for pertussis, suspected cases must exhibit a cough lasting at least two weeks, creating a prolonged period during which patients may be infectious before a diagnosis is confirmed. Mild cases in adolescents and adults are particularly susceptible to being overlooked or misdiagnosed.

Studies examining the sensitivity and specificity of this case definition highlight its limitations. For instance, one study involving 328 symptomatic children aged 6–14 found that, in a non-outbreak setting, the case definition had a high sensitivity of 95.2% but a low specificity of 15% when applied to cases with a cough lasting at least two weeks alongside one or more WHO clinical criteria. As the number of clinical findings increased, sensitivity decreased while specificity improved. Post-tussive emesis had the greatest impact on increasing specificity.

Bacterial gene mutations

The resurgence of Bordetella pertussis has been linked to the evolution and accumulation of mutations that enhance its pathogenicity, immune evasion, and drug resistance. These genetic changes enable the bacteria to survive more effectively in immunised populations, contributing to its re-emergence.

The resurgence of Bordetella pertussis is closely linked to genetic changes in key surface proteins, such as pertussis toxin (PT) and pertactin (PRN). The ptxP3 strain, which has a heightened capacity to secrete PT, has largely replaced the ptxP1 strain worldwide and is now detected in 11 countries across Asia, Europe, and the Americas. This shift is associated with increased respiratory infections, hospitalisations, and deaths.

Variations in the PT gene—particularly in the A and B subunits—have enabled the bacterium to evade immune responses and alter receptor binding. Additionally, Australia has reported strains with deletions in filamentous haemagglutinin (FHA), further reducing vaccine efficacy.

Drug resistance is another critical factor, largely due to mutations such as the 23S rRNA A2047G mutation, which confers resistance to erythromycin. These genetic adaptations collectively support the pathogen’s survival in vaccinated populations and contribute to its resurgence.

Global Travel and Disease Importation

The resurgence of measles and pertussis is not limited to countries with low vaccination rates. Global travel and mobility significantly contribute to the spread of these diseases. For example, measles outbreaks have often been linked to international travellers who contract the virus abroad and then bring it back to countries such as the UK and Australia, where unvaccinated populations create vulnerable pockets.

In recent years, the UK has experienced multiple measles outbreaks linked to imported cases, often originating from countries with lower vaccination rates. These imported cases have highlighted gaps in immunity within certain communities, amplifying the risk of local transmission.

Pertussis can also be imported; however, it spreads more readily within domestic populations, largely due to waning immunity among older age groups. With international travel rebounding post-pandemic, the risk of importing infectious diseases like pertussis remains a significant public health concern.

Changes to vaccine schedule: Acellular vs whole cell vaccines

Immunity against pertussis diminishes over time, whether acquired through natural infection or vaccination. Research indicates that vaccine-induced antibodies persist for 4–12 years, while immunity from natural infection lasts between 4 and 20 years.

In 2004, the UK introduced the acellular pertussis (aP) vaccine, replacing the whole-cell pertussis (wP) vaccine due to its association with fewer adverse events. However, data suggests that the acellular vaccine primes the immune system differently, eliciting a distinct immunological response compared to the whole-cell vaccine. This shift may have contributed to the resurgence of pertussis.

The wP vaccine stimulates a stronger host-adaptive immune response, providing more robust protection against Bordetella pertussis nasal colonisation. In contrast, the aP vaccine primarily induces a weaker Th2 immune response, which, while effective in preventing symptomatic illness, is less efficient at preventing transmission. This reduced efficacy in blocking nasal colonisation may allow for asymptomatic spread, contributing to the ongoing circulation of pertussis in the population.

How can we reduce the numbers and prevent further outbreaks?

Early recognition of cases and population-level surveillance

When disease incidence is low, clinical diagnosis becomes less reliable, highlighting the importance of laboratory testing to confirm or rule out suspected cases. Clinicians play a crucial role in identifying measles and pertussis cases through careful clinical assessment and epidemiological context. Robust virological and epidemiological surveillance is essential for identifying index cases and supporting thorough contact tracing, which is vital for maintaining control over disease spread. By detecting transmission chains early, healthcare providers can implement timely interventions to reduce further transmission and protect public health.

Post Exposure Prophylaxis

Measles

Immunocompromised individuals, including pregnant women and infants, should receive prompt post-exposure prophylaxis to reduce the risk of disease spread and complications. Depending on the individual’s circumstances, this may involve either the administration of the MMR vaccine within 72 hours of exposure or intravenous immunoglobulin within six days.

Although post-exposure prophylaxis is recommended in cases of measles, these interventions have limited effectiveness, making surveillance a priority. Notably, the MMR vaccine offers lifelong protection against measles, underscoring the importance of vaccination for long-term immunity.

Pertussis

Antibiotic chemoprophylaxis with a macrolide, such as erythromycin, should be given to all close contacts if the index case’s cough began within the previous 14 days. Chemoprophylaxis is most effective when administered promptly following exposure. Additionally, unimmunised or partially immunised children under 10, pregnant women who have not received a pertussis dose during their current pregnancy, and healthcare workers without a booster in the past five years should all receive the pertussis vaccine to bolster protection against infection.

Reporting and Vaccination of Healthcare Workers

Both measles and whooping cough are notifiable diseases, meaning that all suspected cases must be reported to the local Public Health Unit or health protection team. This ensures timely public health management, including contact tracing, isolation of positive cases, and administering post-exposure prophylaxis when necessary. Prompt reporting and intervention are essential to control the spread of these highly contagious infections.

Healthcare workers face a heightened risk of infection and can act as potential sources of transmission. In outbreak scenarios, it is essential to assess healthcare workers and isolate them when necessary to prevent further spread. Vaccination among healthcare workers should be prioritised as a key preventive measure to reduce the risk of disease transmission within healthcare settings and protect staff and patients.

Clear history taking

In all suspected cases, test.

Remember, family members and siblings may also be at risk or not vaccinated.

Early testing in Measles

Measles diagnosis can be confirmed through serologic testing or polymerase chain reaction (PCR) assays, while pertussis is typically diagnosed via PCR testing or culture of nasopharyngeal samples. Healthcare providers should maintain a low threshold for testing, particularly during outbreaks or in patients with known exposure, to ensure prompt identification and management of these contagious diseases.

Measles is a single-stranded RNA virus with fewer than three genotypes circulating globally, each predominantly found in specific geographical regions. These genotypes allow for tracking of transmission patterns and origins, aiding in outbreak investigation and control efforts.

In most countries, oral fluid (OF) testing for IgM, IgG, and RNA is the preferred sample type for measles testing and surveillance. In paediatric settings, OF sampling is particularly advantageous as it is easier to collect and less invasive than serum or swab samples, making it more suitable for children. Oral fluid (OF) samples can also be used for genotyping to identify imported measles cases, which is valuable for surveillance and tracking transmission. Additionally, OF testing allows for reliable diagnosis or exclusion of measles and can help determine whether a case is primary or breakthrough, enhancing the accuracy of public health responses.

In the initial days following rash onset, oral fluid (OF) testing for IgM antibodies is more sensitive and specific than serum testing for measles. On the first day of rash appearance, around 50% of OF samples test positive for IgM antibodies, increasing to over 90% by day three.

The timing of rash onset is especially critical for serum sampling, as IgM antibodies may not reach detectable levels within the first three days due to lower circulating levels of virus-specific IgM. In patients with confirmed measles, serum sampling becomes highly sensitive from day four onwards. One study demonstrated that from day four to day eleven, 100% of serum samples collected from confirmed cases were IgM positive, underscoring the importance of timing in sample collection for accurate diagnosis.

Early testing in Pertussis

Bacterial culture was once considered the gold standard for diagnosing pertussis due to its nearly 100% specificity. However, its sensitivity varies widely, from 12% to 60%.

Culture is more sensitive when performed within the first few weeks of cough onset. However, sensitivity decreases as the illness progresses to the paroxysmal stage—characterised by distinctive coughing fits—increasing the likelihood of false-negative results. This timing sensitivity limits the culture’s effectiveness in later stages, highlighting the need for alternative diagnostic methods, particularly PCR, in confirming pertussis cases as symptoms evolve.

PCR testing is recommended for pertussis diagnosis if the cough has been present for 0–4 weeks. Although, like culture, its sensitivity declines over time, PCR remains a more sensitive method, with sensitivity rates ranging from 70% to 99%, especially in young children and infants. Ideally, nasopharyngeal swab (NPS) samples should be used for PCR testing, but if NPS is not feasible, throat swabs or nasopharyngeal aspirates are acceptable alternatives.

For cases where the cough has persisted for 2–12 weeks, serology testing to detect anti-pertussis IgG (anti-PT IgG) can be considered. However, in infants and younger children, maternal antibodies or recent vaccinations may affect results, which can confound the interpretation of IgG levels.

Vaccination counselling

In busy clinical settings, healthcare professionals may sometimes overlook discussing immunisation with patients who are not fully vaccinated. However, given the rising incidence of vaccine-preventable diseases, providers must engage with parents who choose not to vaccinate their children, informing them about the associated risks. While healthcare professionals cannot force parents to vaccinate, they are responsible for providing comprehensive information, including recent trends and case data, to support informed decision-making. Educating parents in this way can help them understand the potential consequences of non-vaccination, both for their children and the broader community.

National initiatives

Conclusion

The rise in measles and pertussis cases in the UK underscores a significant public health challenge. This increase is driven by many factors, including declining vaccination rates, diagnostic challenges, waning immunity, and the disruptions caused by the COVID-19 pandemic.

Infants under one year who have not yet completed their primary vaccination schedule are particularly vulnerable to these infections. Addressing this resurgence requires a comprehensive public health response focusing on improving vaccination coverage, enhancing diagnostic practices, and closing immunity gaps, especially in at-risk populations.

Addressing these challenges requires both individual and collective action.

At the individual level, healthcare providers can make a significant impact by educating families on timely vaccinations, recognising early symptoms, and ensuring prompt reporting of suspected cases. Vaccination campaigns targeting under-immunized communities, along with post-exposure prophylaxis for high-risk contacts, are potential steps to break the chain of transmission.

National initiatives, such as catch-up programmes, public health campaigns, improved surveillance, and support for maternal vaccination programs, are essential to restoring herd immunity and preventing further outbreaks.

By focusing on these strategies, we can protect vulnerable populations and prevent the resurgence of these vaccine-preventable diseases.

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