COVID-19 cases tend to rise in predictable waves driven by a combination of factors: new variants that sidestep existing immunity, the natural fading of protection from vaccines and prior infections, and seasonal conditions that favor viral spread. In early 2026, CDC surveillance shows a test positivity rate of 4.7% and national wastewater viral activity at “low” levels, but even modest upticks can feel alarming. Understanding the forces behind these surges helps put the numbers in context.
New Variants Outrun Your Immune Defenses
The virus never stops evolving, and each new dominant lineage is better at slipping past antibodies you built from vaccines or past infections. As of mid-February 2026, CDC genomic surveillance shows the XFG family of subvariants accounts for roughly 65% of circulating virus in the United States. XFG alone makes up about 29% of sequenced cases, with offshoots like XFG.2.5.1 (16%), XFG.1.1 (9%), XFG.14.1 (7%), and XFG.6 (4%) filling out the landscape. NB.1.8.1, a separate lineage, represents about 21%.
What makes these variants spread so effectively is a pattern the virus has followed since Omicron: accumulating mutations in the spike protein, the part of the virus your immune system recognizes most readily. Earlier research on Omicron-era variants mapped out how specific changes to the spike protein’s receptor-binding domain let the virus dodge neutralizing antibodies. Mutations at key positions disrupted the binding sites targeted by several classes of antibodies, essentially rendering portions of your immune memory less useful. Current variants carry their own updated set of these escape mutations, which is why a previous infection from even six months ago offers limited protection against reinfection.
Vaccine Protection Fades Faster Than You’d Expect
The updated KP.2 vaccines rolled out for the 2024-2025 season, and long-term follow-up data published in Nature Communications paints a clear picture of how quickly their protection declines. At 60 days after vaccination, effectiveness against any lab-confirmed infection was about 31%. By 90 days, it dropped to roughly 26%. At 120 days, it sat around 22%. Over a mean follow-up of 172 days (about six months), effectiveness against infection fell to just 16.6%.
Protection against the outcomes that matter most held up better, though it still declined. Effectiveness against death started at 75% at 60 days and fell to about 63% by 120 days. That’s a meaningful gap: the vaccines continue to reduce your risk of severe illness and death well after their ability to prevent infection has largely worn off. But from the standpoint of case counts, the majority of vaccinated people lose most of their infection-blocking protection within a few months. In a population where most people were vaccinated or boosted months ago, this creates a growing pool of susceptible individuals, which is exactly the condition a new wave needs to take off.
Seasonal Patterns Still Shape Surges
COVID-19 follows seasonal rhythms similar to flu and other respiratory viruses, though its waves can strike at any time of year. The key environmental driver is indoor humidity. Research published in PLOS One found that each 1 percentage-point increase in relative humidity was associated with a 0.9% reduction in new cases. Temperature had a weaker, statistically less reliable effect, with each 1°C increase linked to roughly a 2.7% fewer cases.
The practical takeaway: during colder months, people spend more time indoors where air is heated and dried out. Low indoor humidity helps respiratory droplets evaporate into smaller particles that hang in the air longer and travel farther. It also appears to impair the mucous membranes in your nose and throat that serve as a first line of defense. Combine that with holiday gatherings, school terms, and increased use of public transit, and you get the behavioral and environmental cocktail that reliably pushes cases upward in winter and early spring. As humidity rises and people shift back outdoors, transmission typically slows.
Reported Numbers Undercount Real Infections
Most people who catch COVID today test at home or don’t test at all, which means official case counts capture only a fraction of actual infections. Wastewater surveillance offers a more honest picture of community spread because it measures viral material shed by everyone, regardless of whether they sought a test. As of the last week of February 2026, CDC’s National Wastewater Surveillance System rated national viral activity at 3.20 on its scale, which falls in the “low” category (the scale runs from “very low” below 2.0 up to “very high” above 7.8).
At the same time, emergency department visits diagnosed as COVID-19 held steady at 0.6% of all visits, and the hospitalization rate was 0.8 per 100,000 people, down from 1.2 the previous week. Test positivity stood at 4.7%, slightly down from 5.1% the week before. These numbers suggest that while the virus is circulating, the current wave is modest compared to earlier peaks. Still, even a “low” baseline in wastewater likely represents tens of thousands of daily infections nationally that never appear in any official tally.
Symptoms With Current Strains
The core symptom list hasn’t changed dramatically. The CDC lists fever or chills, cough, sore throat, congestion or runny nose, fatigue, muscle aches, and headache as the most common signs. Loss of taste or smell, once a hallmark of earlier variants, still appears but is less frequently reported with newer lineages. Shortness of breath, nausea, vomiting, and diarrhea remain possible. Symptoms typically show up 2 to 14 days after exposure, and the CDC notes that symptom profiles can shift with new variants and may differ based on vaccination status. Many vaccinated people who get infected describe something resembling a bad cold that lasts three to seven days.
Antivirals Still Work, but Timing Is Critical
For people at higher risk of severe illness, oral antiviral treatment remains effective, but only if started within five days of a positive test. A large study comparing the two available options found that Paxlovid significantly outperformed molnupiravir across all age groups. Among adults 18 to 59, Paxlovid cut the risk of death by about 52%. For those 60 to 79, molnupiravir reduced mortality risk by 35% and severe disease by 31%, but Paxlovid performed better still. In people 80 and older, the advantage of Paxlovid over molnupiravir was even more pronounced. Crucially, neither drug showed a significant benefit when prescribed more than five days after infection. If you’re over 60 or have conditions that raise your risk, getting tested early enough to start treatment within that window matters more than which specific variant you caught.
Why Waves Keep Coming
Each surge is the product of these forces converging. A new variant family like XFG emerges with enough immune-escape mutations to reinfect people. Months pass since the last vaccine update, and antibody levels drop. Winter pushes people indoors into dry, poorly ventilated spaces. The virus finds millions of newly susceptible hosts and spreads efficiently until enough people have been recently infected or the season shifts. Then cases fall, immunity wanes again, another variant takes hold, and the cycle restarts.
This pattern is unlikely to produce the massive, health-system-straining waves of 2020 or 2021, because the population now carries layers of immune memory from multiple exposures and vaccinations. That background immunity keeps hospitalizations and deaths far lower relative to case counts than in the pandemic’s early years. But it also means periodic surges are a permanent feature of living with this virus, much like annual flu seasons, driven by the same interplay of viral evolution, fading immunity, and human behavior.