COVID-19 Six Years On: Vaccines Changed the Crisis, but the Risks Did Not Disappear

Six years after SARS-CoV-2 emerged, vaccination has sharply reduced deaths and hospital pressure, but the remaining burden is more concentrated, complex and difficult to track. A new review argues that the next phase of COVID policy must focus less on raw case numbers and more on vulnerability, long-term illness and health-system readiness.

Titled "Six Years of COVID-19: Lessons from Epidemiology, Vaccination Campaigns, Clinical Risk Stratification, and Thromboembolic Surveillance," the narrative review published in the journal Vaccines integrates epidemiology, vaccination, geriatric risk, biomarkers, thromboembolic complications, long COVID and future surveillance. It is not a new clinical trial or meta-analysis. Instead, it synthesises major findings from randomised trials, large observational studies, meta-analyses and international guidelines to ask what six years of pandemic experience should change in public health practice.

The review holds that vaccination transformed COVID-19 from a mass mortality emergency into a more manageable but still consequential disease. Yet the remaining burden increasingly falls on older adults, frail patients, immunocompromised people and those living with multiple chronic conditions. The disease has also revealed how infection, inflammation, vascular injury, clotting abnormalities and long-term disability can remain connected well beyond the acute phase.

The Emergency Faded, but the Risk Became More Unequal

The early pandemic was defined by widespread population vulnerability. There was little immunity, limited testing, no vaccine and few proven treatments. Hospitals faced abrupt surges, and public policy relied heavily on lockdowns, distancing, masking and travel restrictions.

The epidemiological landscape changed as vaccination, previous infection and improved clinical care built population immunity. The review notes that large-scale vaccination sharply reduced severe disease, intensive care admission and mortality, weakening the once-close relationship between infection numbers and hospital deaths.

However, reduced average severity does not mean equal safety. COVID-19 now behaves less like a uniform population-wide threat and more like a risk concentrated in medically vulnerable groups. Frail older people, patients with cardiovascular or respiratory disease, people with cancer, transplant recipients and those receiving immunosuppressive therapy remain more likely to experience severe outcomes.

During the emergency, governments often measured risk through daily cases and national mortality. Those indicators are less useful when widespread circulation produces mostly mild disease in some groups but serious illness in others. The review supports a transition from population-wide emergency management to targeted protection. Surveillance systems should identify not only how much virus is circulating, but who is being hospitalised, which variants are driving severe disease, how immunity is waning and which communities lack adequate access to vaccination or care.

This is particularly important in low- and middle-income countries, where age structure, disease burden, health access and vaccine availability differ widely. A model designed around sophisticated hospital databases and genomic systems may be difficult to reproduce in settings with limited laboratory capacity, fragmented records and shortages of health workers.

The lesson is not that lower-resource countries should copy high-income surveillance architecture. It is that surveillance must become more selective and useful. Even modest systems can prioritise severe respiratory admissions, outbreaks in vulnerable populations, vaccine uptake, excess mortality and sentinel testing.

Vaccines Won the First Battle; Protection Is Now a Moving Target

The review describes rapid COVID-19 vaccine development as one of the defining achievements of modern medicine. Multiple platforms moved from sequencing to emergency use within a year, supported by public investment, international research networks and accelerated regulation.

Real-world evidence then confirmed that vaccination substantially reduced hospitalisation, intensive care demand and death. Early mRNA vaccines produced very high protection against severe outcomes during the ancestral and early variant periods, although protection against infection declined as immunity waned and Omicron evolved.

COVID-19 vaccines are no longer best judged by whether they prevent every infection. Their main public health value lies in preventing severe disease, protecting high-risk populations and reducing pressure on health systems.

The review traces how uniform mass campaigns gradually shifted toward risk-based booster strategies. Older adults, residents of care facilities, immunocompromised patients and people with serious chronic disease often require more frequent or specially timed protection. Booster doses restored protection against severe outcomes, especially in these groups, but important uncertainties remain over how often they should be given and how long protection lasts.

This creates a more complicated policy challenge than the first vaccine rollout. Emergency campaigns had a clear objective: vaccinate as many people as possible as quickly as possible. Long-term programmes require decisions about timing, variant updates, cost-effectiveness, public communication and priority groups.

They also require governments to rebuild trust. Vaccine hesitancy, misinformation and declining public attention can undermine targeted campaigns even when the scientific rationale is strong. Communication must therefore explain why recommendations differ by age or health status and why updated vaccines may still be useful even when they do not fully block transmission.

For the Global South, the issue is not only public confidence but supply and delivery. Many countries continue to depend on external procurement, uneven cold-chain capacity and donor-funded programmes. A shift toward periodic boosters could deepen inequalities unless global financing and manufacturing systems support reliable access.

The next generation of vaccine policy should therefore combine precision with equity. More targeted recommendations can improve efficiency, but only if those at highest risk can actually obtain the vaccine.

The Hidden Burden Runs Through Frailty, Clotting and Long COVID

The review insists that COVID-19 cannot be understood as a respiratory disease alone. Severe infection can involve endothelial dysfunction, inflammation, platelet activation and abnormal coagulation. These processes help explain why venous thrombosis, pulmonary embolism and microvascular clotting became prominent features of hospitalised COVID-19.

D-dimer emerged as an important prognostic marker, with elevated levels associated with severe illness, respiratory failure, thrombotic events and mortality. Age-adjusted thresholds may help clinicians avoid unnecessary imaging in older patients, whose baseline levels are often higher.

However, the review cautions against simplistic treatment. Intensified anticoagulation may benefit selected patients, yet universal escalation can increase bleeding risk. The policy lesson is personalised assessment, not blanket intervention.

Frailty is similarly important. Chronological age alone cannot explain why two people of the same age experience very different outcomes. Frailty captures reduced physiological reserve, functional dependence and vulnerability to stress. It may predict deterioration, prolonged hospitalisation and death more effectively than age alone in some clinical settings.

Health systems often rely on age cut-offs because they are easy to administer. Yet age-based rules may overlook robust older adults while missing younger patients with severe multimorbidity. Incorporating frailty and functional status could improve vaccination priorities, treatment decisions and discharge planning.

The burden also extends beyond hospitalisation. Long COVID can involve fatigue, breathlessness, cognitive problems, sleep disturbance, chest pain, autonomic symptoms and reduced quality of life. Symptoms may fluctuate and persist for months or years. Its mechanisms remain uncertain, with possible roles for viral persistence, immune dysregulation, endothelial injury and chronic inflammation. The lack of standard definitions and proven therapies complicates diagnosis, service planning and disability policy.

For developing countries, long COVID creates a particularly difficult challenge. Dedicated clinics, multidisciplinary rehabilitation and advanced diagnostics are costly. Yet ignoring the condition risks shifting a hidden burden onto households, primary care systems and labour markets.

Governments may need tiered models of care: basic recognition and screening in primary care, referral pathways for severe cases and integration with rehabilitation, mental health and chronic disease services. The review's core message is that the burden of infection does not end when a test becomes negative.

The Next Pandemic Will Test What Governments Chose to Keep

The most important policy question is no longer whether countries can respond to a pandemic emergency. It is whether they will preserve the capabilities built during one. The review argues that future surveillance should move beyond case counting and integrate genomic sequencing, wastewater monitoring, vaccination data, hospital use, biomarkers, long-term outcomes and population vulnerability.

The ambitious vision promises earlier detection, more targeted intervention and better allocation of scarce resources. Artificial intelligence and predictive analytics could help identify high-risk patients, anticipate surges and guide vaccination programmes. But digital optimism must be tempered by governance concerns. Surveillance systems require interoperable data, privacy protections, trained personnel and sustained financing. Algorithms trained mainly on high-income populations may perform poorly elsewhere. Weak data quality can produce false confidence rather than better decisions.

The review also has methodological limits. As a narrative review, it does not follow the full procedures of a systematic review or meta-analysis. Study selection may be influenced by publication bias, and the underlying literature varies widely by population, variant, healthcare setting and outcome definition. Most available evidence also comes from countries with strong surveillance systems, limiting its direct applicability to lower-resource settings.

These limitations do not weaken the review's key strategic insight. Global preparedness cannot rest on evidence generated largely in wealthy health systems. Future research must include more data from Africa, Asia and Latin America, particularly on vaccine durability, long COVID, frailty, repeated infection and affordable surveillance models.