Starting a Biologic: What It Means for Infection Risk

Biologics changed the treatment of rheumatoid arthritis, psoriasis, inflammatory bowel disease and several other conditions. They work by switching off a specific part of the immune system, and that is precisely why they change your relationship with infection.

The risk is real, manageable and frequently overstated by the internet. What matters is knowing which infections your particular drug makes more likely, and what to do about the few that are time-critical.

Why these drugs are different from broad immune suppression

Steroids suppress the immune system generally. A biologic blocks one messenger or one cell type, which makes it more effective for the disease and gives it a more predictable pattern of infection risk.

What different targets tend to mean

Drug family	What it blocks	The infection to know about
TNF inhibitors	A key inflammatory messenger	Reactivation of latent tuberculosis; also fungal infections
B-cell depleting therapies	Antibody-producing cells	Hepatitis B reactivation; lower antibody response to vaccines
IL-17 and IL-23 blockers	Other inflammatory messengers	Candida (thrush) more than anything else
IL-6 blockers	An inflammatory messenger	CRP is suppressed, so it cannot be used to detect infection
JAK inhibitors	An intracellular signalling pathway	Shingles, at a higher rate than most
Complement inhibitors	Part of innate immunity	Meningococcal infection; vaccination is mandatory beforehand

That IL-6 row deserves emphasis. On those drugs, CRP is artificially low even during a significant infection — so the number that would normally raise the alarm does not. Our article on what CRP and ESR mean covers the test; on these drugs, symptoms carry the weight instead.

What gets screened before you start

The screening is not a formality, and it is the reason infectious disease is often involved before a biologic starts at all.

1. Latent tuberculosis, by skin or blood test and usually a chest X-ray. If positive, treatment starts before the biologic; see latent tuberculosis
2. Hepatitis B, including the core antibody, because dormant infection can reactivate dangerously; see hepatitis B
3. Hepatitis C and HIV in most protocols
4. Vaccinations brought up to date first, because live vaccines become difficult afterwards; see vaccines when your immune system is suppressed
5. Any active infection, including dental — see when a dental infection spreads
6. Travel and residence history, which brings endemic fungal infections into view; see fungal infections

Living with it

• Keep vaccinations current, using non-live vaccines; flu annually, and shingles matters particularly on JAK inhibitors
• Treat dental problems promptly rather than waiting them out
• Take food safety more seriously than you used to
• Tell any clinician treating you that you are on a biologic — it is not on every record
• Expect the drug to be paused during a significant infection, and resumed after
• Discuss travel in advance, because both vaccines and destinations may need planning; see travel vaccines

What the risk actually looks like in numbers

Precise figures vary by drug, by disease and by study, which is why this article describes shape rather than percentages. A few things are consistent across the evidence and worth holding onto.

• Serious infection is uncommon; most people on a biologic never have one
• The commonest problems are ordinary: chest infections, skin infections, urinary infections
• Risk is highest in the first months and falls as steroids come down
• Combining a biologic with steroids raises risk more than the biologic alone
• Age and other conditions influence the total more than the drug choice does
• Untreated active inflammatory disease carries its own infection risk, which is the comparison that matters

That last point is the one most often left out of the conversation. The alternative to treatment is not a neutral baseline.

Surgery, travel and dental work

Three situations come up repeatedly and are easier handled in advance than improvised.

• Planned surgery. Most biologics are paused around an operation, timed to the drug’s dosing interval. That is a joint decision between the surgeon and the prescriber, and it needs a few weeks of notice; see what to watch for after surgery
• Travel. Live vaccines are generally out, which affects yellow fever in particular. Raise it months ahead, not weeks; see travel vaccines and fever after travel
• Dental work. Routine treatment is fine and should not be delayed. An active dental infection is the thing to deal with before it becomes the problem described in when a dental infection spreads

The first months, and what settles

Infection risk is not evenly spread over time. It is generally highest in the first months of treatment, partly because that is when reactivation of a dormant infection happens, and partly because it often overlaps with the highest steroid doses.

It usually settles as the steroid comes down and the underlying disease comes under control. That is worth knowing at the start, when the list of precautions feels longest and most permanent.

What to watch for, and when to call

The general rule is that an infection on a biologic deserves a lower threshold and an earlier call than the same infection would otherwise.

• Fever, particularly with rigors or shaking chills
• A cough lasting more than a couple of weeks, or any weight loss or night sweats, which raises the question of tuberculosis
• A painful rash in a band on one side of the body, which is shingles and is time-critical
• Spreading redness or a wound that is not healing; see cellulitis
• Persistent thrush in the mouth or throat
• Any of the warning signs of sepsis
• Feeling unwell in a way you cannot explain, which on these drugs is worth acting on

None of this means the drug is the wrong choice. Untreated inflammatory disease carries its own infection risk, and for most people the balance favors treatment clearly. It means the monitoring exists for a reason. You can reach us through the contact page.

Frequently asked questions