Epidemiology

In the UK, before newborn screening programmes for SCID were available, the minimum prevalence for SCID was 0.26/100,000.[6]

In the US, newborn screening for SCID is routinely carried out across all states using the T-cell receptor excision circle assay. One study reporting data from the screening programme in California (carried out between 2010 and 2017, and involving approximately 3.2 million newborns) found the incidence of SCID to be approximately 1 in 65,000 live births (95% CI: 1 in 51,000 to 1 in 90,000 live births).[7]

Before routine screening the incidence of SCID was estimated at around 1 in 100,000 live births, so it is likely that many infants with SCID might not have been diagnosed.

X-linked recessive SCID (caused by mutations in the gene encoding the common gamma chain, located on the X chromosome) is the single most common form of SCID in the US.[8] Historically, it has been reported in up to 50% of cases from referral centres that treat SCID.[8][9][10][11] However, one study reporting data from 11 newborn screening programmes for SCID in the US (carried out between 2008 and 2013, and involving approximately 3 million newborns) found X-linked recessive SCID in approximately 19% of infants with SCID.[9] The reduced proportion of X-linked recessive diagnosis arises from increased ascertainment of autosomal-recessive SCID by population-based newborn screening.[9]

The most commonly reported autosomal-recessive SCID is adenosine deaminase deficiency, accounting for approximately 16% of SCID cases overall.[8] Defects in the alpha-chain of the receptor for interleukin-7 account for approximately 10% of all SCID cases.[8] Mutations in the gene encoding for Janus kinase 3 account for approximately 6.5% of SCID cases.[8] Mutations in the recombination activating genes (RAG1 and RAG2) account for up to 3% of all SCID cases.[8] The reported incidence of autosomal recessive SCID is likely to increase with the continued uptake of population-based newborn screening.

The incidence of autosomal recessive SCID is higher in populations with high rates of parental consanguinity.[12][13][14][15] In Saudi Arabia, SCID incidence of 1 in 2,906 live births has been reported.[14]

Athabascan-speaking Native American people have high rates of autosomal-recessive SCID caused by defects in a DNA repair protein called Artemis, which is encoded by the DNA cross-link repair 1C (DCLRE1C) gene.[15] Incidence of this form of SCID is reported to be approximately 52 per 100,000 live births in this population.[15]

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