What Is the Role of Nanoparticles in COVID Vaccines?

Lipid nanoparticles, or LNPs, are the tiny fat-based shells that carry messenger RNA into your cells in the Pfizer-BioNTech and Moderna COVID-19 vaccines. Without them, the fragile mRNA instructions for building the spike protein would be shredded by enzymes in your body within minutes. But these nanoparticles do more than just protect their cargo. They also help shuttle it across cell membranes, and research now shows they contribute to the immune response itself, functioning partly as built-in adjuvants.

Why mRNA Needs a Protective Shell

Messenger RNA is chemically delicate. Your body is full of enzymes called nucleases whose job is to break down stray RNA. Injecting naked mRNA into muscle tissue would be largely pointless because the molecules would degrade before reaching the inside of a cell. The solution that made mRNA vaccines practical was encapsulating the mRNA inside lipid nanoparticles. These particles form a stable nanostructure in which mRNA molecules sit in the interior core, held in place by electrostatic interactions with the surrounding lipids. That structural arrangement shields the mRNA from nuclease degradation in body fluids and keeps the particle intact long enough to reach target cells.¹

The concept did not appear overnight. Researchers spent roughly six decades developing lipid-based delivery systems for nucleic acids, starting with early lipoplexes made from positively charged lipids and evolving toward the more sophisticated ionizable-lipid nanoparticles used today.² The COVID-19 pandemic was the moment this long-running research paid off at a global scale.

The Four Ingredients Inside an LNP

Each lipid nanoparticle in a COVID mRNA vaccine is built from four types of lipid, and each has a distinct job.³

Ionizable lipids: These are the star players. At the neutral pH of your blood and tissue fluid, they carry no charge, which helps the particle avoid unwanted interactions. But once the particle is swallowed by a cell and lands inside an acidic compartment called an endosome, these lipids pick up a positive charge. That charge shift is what lets the mRNA escape into the cell’s interior, where it can be read by ribosomes to produce protein.
Helper lipids: Also called neutral or structural lipids, these stabilize the particle’s membrane and influence how easily it fuses with cell membranes.
Cholesterol: Borrowed from the same molecule circulating in your blood, cholesterol fills gaps between the other lipids, making the particle sturdier and less likely to fall apart before reaching a cell.
PEG-lipids: A lipid attached to polyethylene glycol, a water-loving polymer. The PEG coating acts like a stealth layer, preventing the particles from clumping together and slowing their clearance by immune cells long enough for them to reach target cells.

The Pfizer-BioNTech vaccine uses an ionizable lipid called ALC-0315, while Moderna’s vaccine uses SM-102. Both were specifically designed with ester bonds in their lipid tails so the body can break them down and clear them relatively quickly after they have done their job. Compared with an older ionizable lipid called MC3, SM-102 was cleared faster from the injection site and was better tolerated in animal studies.⁴ The biodegradable design was intentional: an ideal drug carrier disappears once its purpose is served.

How LNPs Get mRNA Into Your Cells

After injection into your upper arm muscle, the lipid nanoparticles encounter cells in the surrounding tissue and are taken up through several natural processes that cells already use to absorb material from their environment. Once inside, the particle typically ends up trapped in an endosome, a small acidic bubble within the cell. This is where the ionizable lipid earns its name. The low pH inside the endosome causes the ionizable lipids to become positively charged, which destabilizes the endosomal membrane and lets the mRNA slip out into the main body of the cell.⁵ From there, ribosomes read the mRNA and build copies of the SARS-CoV-2 spike protein, which your immune system then learns to recognize.

Not all particles stay at the injection site. Research on different LNP formulations has shown that their composition influences where they travel in the body. Some formulations remain largely in the muscle, while others enter the bloodstream and preferentially accumulate in organs like the liver or lungs.⁶ For COVID vaccines delivered into the arm, the particle design is optimized to keep most of the action local, but some systemic distribution does occur. Pharmacokinetic studies of ALC-0315 administered intravenously in animals found that the ionizable lipid accumulated most in the liver and spleen, while mRNA concentrations peaked in the spleen and heart, showing that each component has its own distribution profile.⁷ It is worth noting that intravenous injection is not how these vaccines are given to people; the intramuscular route produces a different distribution pattern with more localized activity.

LNPs as Built-In Immune Boosters

For years, the nanoparticle shell was thought of as just a delivery truck. Newer research tells a more interesting story. Empty lipid nanoparticles, those containing no mRNA at all, can stimulate immune responses on their own, acting like an adjuvant. In experiments comparing empty LNPs to a standard research adjuvant, the LNP group generated higher neutralizing antibody titers and stronger T-cell responses. Critically, this adjuvant effect depended on the ionizable lipid component. When the ionizable lipid was swapped for a different type of charged lipid, the immune-boosting effect essentially vanished.⁸

This finding has practical implications. It means the ionizable lipids in COVID vaccines are pulling double duty: delivering the mRNA and helping to kickstart the immune response that makes the vaccine effective. A growing body of work has focused on understanding exactly how ionizable lipids trigger innate immune activation and inflammation, because that knowledge could be used to fine-tune future vaccines for either stronger or gentler immune reactions depending on what is needed.⁹

Side Effects Tied to the Nanoparticle Shell

The same immune-stimulating properties that make LNPs useful as adjuvants also contribute to some of the side effects people experience after vaccination: sore arms, fatigue, fever, chills. The PEG-lipid component in particular has drawn attention. Most people already carry low levels of antibodies against PEG from everyday exposure to products like cosmetics and processed foods that contain polyethylene glycol. Vaccination with PEG-containing LNPs can boost those pre-existing anti-PEG antibodies. In one study of 130 adults, anti-PEG IgG levels rose an average of about 13-fold after Moderna vaccination and about 1.8-fold after the Pfizer-BioNTech vaccine. Anti-PEG IgM increased even more dramatically, roughly 69-fold on average with Moderna. Those elevated anti-PEG antibody levels correlated with increased systemic side effects after the second dose.¹⁰

A small fraction of vaccine recipients experience more serious hypersensitivity reactions or, rarely, anaphylaxis. A study of 291 blood donors found that both mRNA vaccines caused significant rises in anti-PEG IgG and IgM, with greater than 10-fold rises in roughly 10 percent of Comirnaty recipients and all Spikevax recipients tested. The 15 subjects who experienced hypersensitivity reactions had significantly higher anti-PEG antibody levels than those who did not react. The researchers identified a small group of “supercarriers” with extremely high baseline anti-PEG antibodies who may be at elevated risk and suggested that screening for these individuals could help predict who is likely to have a reaction.¹¹

The rare cases of myocarditis reported after mRNA vaccination have also prompted questions about whether LNPs themselves play a role. The observation that a similar adverse event occurred in a phase III trial of the Novavax protein-based vaccine, which uses a different nanoparticle platform entirely, raised the possibility that the lipid nanoparticle shell could be one contributing factor, though the spike protein itself remains the more commonly discussed suspect.¹² Researchers are actively working on lower-inflammatory LNP designs. One group developed a modified lipid nanoparticle using a disulfide-cleavable material that produced less inflammatory cytokine release, less vascular leakiness, and fewer adverse reactions like weight loss and fever in animal studies compared with conventional LNPs.¹³

Not All COVID Vaccine Nanoparticles Are the Same

When people hear “nanoparticles in COVID vaccines,” they usually think of the mRNA vaccines. But the Novavax vaccine (NVX-CoV2373) also uses nanoparticle technology, just a completely different kind. Instead of delivering genetic instructions for your cells to build the spike protein, Novavax delivers the pre-made spike protein itself, assembled into nanoparticles. These recombinant protein nanoparticles are made of trimeric full-length spike glycoproteins and are combined with Matrix-M1, a saponin-based adjuvant derived from tree bark, rather than relying on the nanoparticle shell itself for immune stimulation.¹⁴ For people who were uncomfortable with the idea of mRNA technology or had reactions to PEG-containing formulations, the Novavax option offered a different nanoparticle approach with its own risk-benefit profile.¹⁵

The Cold Chain Problem and Manufacturing Scale

One of the most visible limitations of early mRNA-LNP vaccines was storage. The Pfizer-BioNTech vaccine initially required ultra-cold freezing at around minus 70 degrees Celsius, and even the Moderna vaccine needed standard freezer temperatures. These requirements created enormous logistical headaches, especially for distribution in regions without reliable cold chain infrastructure. The underlying challenge is that lipid nanoparticles can degrade, aggregate, or leak their mRNA cargo when stored for extended periods at warmer temperatures.

Lyophilization, essentially freeze-drying the vaccine into a powder, offers a potential path forward. Researchers have demonstrated that nucleoside-modified mRNA-LNPs can be lyophilized and stored at room temperature for 12 weeks, or at refrigerator temperature for at least 24 weeks, without significant changes to their physical properties. In mouse studies, the freeze-dried formulations maintained both their ability to produce protein and their immunogenicity across those storage windows.¹⁶ If this approach translates to human vaccines, it would eliminate the deep-freeze requirement and radically simplify global distribution.

Manufacturing at scale presents its own challenges. The lipid nanoparticles used in COVID vaccines are produced using rapid-mixing techniques, often involving microfluidic devices that push lipid solutions and mRNA solutions together through tiny channels just tens of micrometers wide. This approach creates particles with highly controlled and reproducible sizes and structures.¹⁷ However, scaling microfluidic production from lab bench to billions of doses has proven difficult. Despite advantages in size control, distribution uniformity, and encapsulation efficiency over bulk methods, translating microfluidic mixing to commercial manufacturing remains an active engineering challenge.¹⁸

Self-Amplifying RNA and New Delivery Routes

The next generation of LNP-based vaccines aims to do more with less. Self-amplifying RNA, or saRNA, encodes not just the antigen but also the molecular machinery needed to copy itself inside your cells. Once delivered by an LNP, a single saRNA molecule can generate many copies of the antigen-encoding mRNA, producing more protein from a smaller initial dose.¹⁹ This could mean lower doses, fewer side effects, and cheaper manufacturing. The first saRNA vaccine, approved in Japan in late 2023 for COVID-19, proved the concept was viable in humans.

Optimizing LNPs for saRNA is not identical to optimizing them for conventional mRNA, because the self-amplifying molecules are much larger. The choice of ionizable lipid and helper lipid matters for both potency and storage stability, and formulations that work well for standard mRNA do not always perform the same way with saRNA. Research comparing different ionizable lipid and helper lipid combinations found that some pairings significantly improved both protein expression and immune responses to a SARS-CoV-2 spike saRNA vaccine, while also balancing storage stability requirements.²⁰

Another promising direction is changing how LNP vaccines are delivered to the body. Current COVID vaccines are injected into muscle, which generates strong systemic immunity but weaker mucosal immunity in the nose and throat, exactly the tissues where SARS-CoV-2 first takes hold. Intranasal delivery of mRNA-LNPs is being explored as a way to generate mucosal antibodies right where the virus enters. In mouse studies, intranasal immunization with spike-encoding mRNA-LNPs triggered significantly elevated IgA antibody responses in nasal secretions compared to controls, and offered more protection against SARS-CoV-2 than the intramuscular route.²¹ If intranasal mRNA-LNP vaccines prove safe and effective in humans, they could provide a needle-free option that also better blocks transmission at the point of infection.

Patent Disputes and Access

The technology underlying COVID mRNA vaccines sits in a complicated patent landscape. Multiple institutions and biotech companies hold overlapping intellectual property claims covering both the lipid nanoparticle delivery systems and the modified mRNA structures that make the vaccines work. These disputes have real consequences beyond courtroom drama: they affect licensing agreements, manufacturing partnerships, and how easily lower-income countries can produce or obtain affordable doses. Moderna, BioNTech, Arbutus Biopharma, Acuitas Therapeutics, and the University of Pennsylvania are among the parties whose patent claims have intersected, sometimes contentiously. The resolution of these disputes will shape not just who profits from the current generation of vaccines, but how freely the underlying LNP platform can be used for future medicines targeting cancer, genetic diseases, and other infectious threats.