rs12252 — IFITM3 IFITM3 rs12252 (c.42T>C)

Splice-region synonymous variant in the innate antiviral gene IFITM3; the G allele (C on coding strand) reduces antiviral protein activity and is strongly associated with severe influenza, COVID-19, and other enveloped virus infections.

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IFITM3 rs12252 — The Viral Gatekeeper Variant

Every cell in your body has a first line of defence against viruses before the adaptive immune system has time to respond. IFITM311 IFITM3
Interferon-Induced Transmembrane Protein 3 — a small membrane protein that acts as a physical barrier preventing enveloped viruses from fusing with endosomal membranes and entering the cell is one of the most potent of these innate antiviral sentinels. When interferon signals arrive (as they do within hours of viral exposure), IFITM3 expression surges, lining the endosomal compartments where influenza, SARS-CoV-2, Ebola, and other enveloped viruses must complete their entry. rs12252 is the most studied genetic variant in this gene — and it carries a striking population frequency divide that helps explain why some ethnic groups have been disproportionately affected by pandemic influenza.

The Mechanism

rs12252 sits at position 320,772 on chromosome 11 (GRCh38). The IFITM3 gene is transcribed from the minus strand, so papers often refer to this variant using coding-strand notation (T/C) — but in genome files, the alleles are A (reference, normal) and G (risk). The G allele corresponds to a c.42T>C substitution that is formally synonymous: both codon forms encode serine at position 14 (p.Ser14=). However, the nucleotide change sits at the first splice acceptor site22 splice acceptor site
The sequence at the 3' end of an intron that signals where to cut and join exons during mRNA processing of IFITM3's intron, and the original hypothesis proposed that the C allele disrupts splicing, producing a truncated IFITM3 protein lacking the N-terminal 21 amino acids (Δ21 IFITM3) — a region containing an endocytic sorting signal essential for correct intracellular localisation.

The exact mechanism remains actively debated. A 2017 study using RNA sequencing of primary cells from CC homozygotes33 RNA sequencing of primary cells from CC homozygotes detected no alternatively spliced transcripts, raising questions about whether the truncated protein actually forms in vivo. An alternative explanation is that the G allele reduces total IFITM3 protein expression rather than producing a truncated isoform — functional assays in lymphoblastoid cell lines from CC individuals show substantially lower IFITM3 protein levels and far greater susceptibility to influenza A infection compared to TT homozygotes. Whatever the molecular basis, the biological and clinical consequence — reduced antiviral IFITM3 activity — is well replicated.

The Evidence

The landmark study by Everitt et al. (2012, Nature)44 Everitt et al. (2012, Nature) used both a mouse knockout model and a human genetic association study. Mice lacking Ifitm3 developed fulminant viral pneumonia from a normally low-pathogenicity influenza strain. In hospitalised UK patients during the 2009 H1N1 pandemic, rs12252-C was significantly enriched — approximately 1 in 17 hospitalised patients carried the CC genotype versus 1 in 300 in the general British population.

The population impact became even clearer in East Asian populations. In the Zhang et al. 2013 Nature Communications study55 Zhang et al. 2013 Nature Communications study, the CC genotype was present in 69% of Chinese patients with severe pandemic H1N1 infection versus 25% in mild cases. The population-attributable risk of severe influenza from rs12252 was estimated at 54.3% in the Chinese population — compared to just 5.4% in Northern Europeans. Because the G allele reaches ~53% frequency in East Asians (versus ~4% in Europeans), what is a rare variant in Europe becomes a common variant in East Asia, with enormous implications for population-level susceptibility.

A 2015 meta-analysis66 2015 meta-analysis pooling 4 studies (445 influenza patients, 3,396 controls) confirmed: the C allele confers OR 2.70 (95% CI 1.86–3.94) for severe influenza specifically. A second independent meta-analysis in 2015 (PMID 25942469)77 meta-analysis in 2015 (PMID 25942469) reached similar conclusions across susceptibility and severity outcomes.

For COVID-19, early Chinese cohort data showed the CC genotype was associated with more severe disease in an age-dependent manner. A 2022 meta-analysis (PMID 35461906)88 2022 meta-analysis (PMID 35461906) found odds ratios of 1.65–5.88 across multiple genetic models for COVID-19 susceptibility and severity. Results across individual cohorts have been mixed — several European cohorts found no significant association, likely because the G allele is too rare there to detect effects in small studies. The evidence is most consistent in East Asian and African-ancestry populations where the allele is common enough to generate statistical power.

IFITM3 also restricts Ebola virus, dengue, HIV, and West Nile virus entry in cell culture, though population genetic association data for rs12252 and these infections are limited compared to the influenza and COVID-19 literature.

Practical Actions

For individuals carrying one or two G alleles, the primary implication is respiratory virus risk management. Annual influenza vaccination takes on heightened importance: the mechanism of rs12252 risk is about viral entry and early innate defence, which is precisely what vaccination bypasses by generating adaptive immunity before exposure. COVID-19 vaccination similarly converts an innate immunity deficit into an adaptive immunity advantage.

Antiviral medications (oseltamivir for influenza, antivirals for COVID-19) act on viral replication — a distinct step downstream of IFITM3 function — and their benefit is not impaired by this variant. This means GG individuals benefit at least as much as others from early antiviral treatment if they develop influenza or COVID-19 symptoms.

Interactions

rs34481144 is the second major functional variant in IFITM3 — a regulatory SNP in the promoter that modulates IFITM3 expression levels. Haplotype studies show that the combination of rs12252 G allele and rs34481144 risk haplotype tracks with COVID-19 mortality ratios across ethnic groups in England better than either SNP alone. The two variants should be interpreted together when full IFITM3 haplotype information is available.

rs3764880 (IFITM3 coding variant) and rs3775291 (TLR3) are in nearby antiviral pathway genes. Combined defects in TLR3-interferon signalling and IFITM3 viral restriction could theoretically compound respiratory virus severity, but direct interaction data for rs12252 with these specific variants are not yet published.

Genotype Interpretations

What each possible genotype means for this variant.

AA Full IFITM3 Activity Normal

Normal IFITM3 function — standard innate antiviral defence

You carry two copies of the A allele (T/T in coding-strand notation), the reference form of IFITM3 at rs12252. Your innate antiviral defence protein functions normally at this locus, with standard ability to restrict enveloped virus entry including influenza and SARS-CoV-2. About 72% of people of mixed global ancestry share this genotype; in European populations it is approximately 93% of people. This result does not mean you have a special advantage — it simply means rs12252 is not reducing your IFITM3 function.

AG Reduced IFITM3 Activity Intermediate

One copy of the risk allele — moderately reduced IFITM3 antiviral activity

Meta-analysis data suggest heterozygote risk falls between the homozygous states. The OR for severe influenza for the combined AG+GG group versus AA is approximately 1.62 (95% CI 1.18–2.22), while the OR for GG versus AA+AG is approximately 2.38 — indicating meaningful heterozygote risk but substantially lower than homozygous risk. For COVID-19, AG individuals appear at intermediate risk in populations where the allele is common enough to study (East Asian, African ancestry).

GG Homozygous Risk — Low IFITM3 Activity High Risk

Homozygous risk allele — substantially reduced IFITM3 antiviral activity; highest risk for severe viral illness

The Everitt 2012 Nature study found that among hospitalized UK H1N1 patients, roughly 1 in 17 were CC homozygotes — versus 1 in 300 in the general British population. In the Wang 2013 Nature Communications study of Chinese patients, the CC genotype was present in 69% of severe cases versus just 25% in mild disease, with an estimated population-attributable risk of 54.3% for severe influenza in the Chinese population. The meta-analytic OR for CC versus CT+TT across studies is approximately 2.38–6.0 depending on the population and severity endpoint. For COVID-19, early cohort data (Beijing Youan Hospital, 2020) found CC genotype independently associated with severity in an age-dependent manner; a 2022 meta-analysis found ORs of 1.65–5.88 across different genetic models. The exact molecular mechanism is debated: the G allele sits at a splice acceptor site and may reduce IFITM3 protein expression or produce a truncated isoform, but RNAseq studies have not confirmed alternative splicing in vivo. What is unambiguous is the functional consequence — substantially reduced IFITM3 antiviral activity.