Explaining varicella, exclusion and post-exposure prophylaxis — OSCE

Candidate instructions (8-minute station)

You are the paediatric registrar in the clinic. A 4-year-old child who has received both varicella vaccine doses has developed a mild rash over the last two days — a handful of vesicles, a slight fever, and some itch. The rash is confirmed as breakthrough varicella. The child is otherwise well and recovering. At home, the family has a 2-week-old newborn niece staying with them, and the child's mother is 28 weeks pregnant. [3]

Your tasks are: [2]

1. Explain the diagnosis of breakthrough varicella in plain language, and reassure the family that the vaccine still did its job. [3]
2. Explain the exclusion plan — that the child should stay home and away from vulnerable contacts until every lesion has crusted. [2]
3. Explain why the newborn niece and the pregnant mother need urgent assessment and post-exposure prophylaxis, and what that involves. [2] [4]
4. Address the father's confusion that a fully vaccinated child caught chickenpox at all. [3]

You are not expected to prescribe doses for the newborn yourself without senior review — flag that the newborn needs same-day assessment because varicella in a young infant can be severe. [4]

Examiner prompt to the actor (father)

"But he's had all his needles — both of them — so how can he have chickenpox? Isn't the vaccine supposed to stop this? And he's barely unwell, so do we really need to keep him home? You're telling me the new baby and my wife, who's pregnant, need to be seen today — but they're not even sick. Surely this is overkill?" [2]

Marking domains

• Frame and explanation (3): explains breakthrough varicella in plain, reassuring language; names that the vaccine made this illness milder and that breakthrough disease is expected, not a failure; sets the expectation that the child is recovering and not in danger. [3]
• Exclusion and public-health plan (3): explains clearly that the child should stay home and away from vulnerable contacts until every lesion has crusted, because the danger is transmission to others, not the child's own recovery; explains that varicella is notifiable and that public health may be in contact. [2]
• Protecting the high-risk contacts (3): explains that the newborn niece (too young to be vaccinated) and the pregnant mother are exactly the contacts we protect — that prompt assessment and post-exposure prophylaxis can prevent or attenuate disease; frames that the newborn needs same-day review because varicella can be severe in young infants, and that the pregnant mother needs immune-status assessment and VZIG if non-immune. [2] [4]
• Communication (1): acknowledges the father's confusion without defensiveness, uses plain language, checks understanding, and does not overwhelm. [3]

Model answer — the explanatory script

"Thank you for coming in. I've looked at the rash, and it is chickenpox — varicella. The good news is that it's a very mild case, and your son is doing well, and I'll explain why that's no accident in a moment." [3]

"Let me take your questions in order, because they're exactly the right ones. First — how can he have chickenpox when he's been fully vaccinated? The varicella vaccine is excellent, but like every vaccine it doesn't prevent one hundred percent of cases. What it does extremely well is prevent the severe form of the disease. So when a vaccinated child does catch it, the illness is mild — a few spots, a slight temperature, and he's already bouncing back. If he hadn't been vaccinated, this would likely have been a much nastier illness with a high fever and spots everywhere. So the vaccine absolutely did its job — it turned a potentially miserable, even dangerous illness into something mild." [3]

"Now, why does he need to stay home, if he's barely unwell? Because even a mild case can still pass the virus on, and the people we're most worried about are the ones in your house who can't handle it the way he can. He's contagious from before the rash appeared until every single spot has dried into a crust. That usually takes about a week. So he stays home from childcare and away from other children — and especially away from the new baby and from your wife — until every spot has crusted over. It's not about his recovery now; it's about not passing it to someone it could really hurt." [2]

"And that brings me to the most important part — the new baby, and your wife who's twenty-eight weeks pregnant. Chickenpox in a tiny newborn can be very serious, because their immune system isn't ready and the virus can spread to their lungs and brain. Your niece is only two weeks old and is too young for the vaccine to protect her yet, so we need to take this seriously. I'd like her seen today, and we'll give her a special protective antibody treatment to help stop the virus taking hold — because in a baby this small we don't wait and see." [2] [4]

"Your wife, being pregnant, is the other priority. Chickenpox during pregnancy can cause a pneumonia that's more serious than usual, and in the first half of pregnancy there's a small risk to the baby. We need to check whether she's already immune — most adults who've had chickenpox before are, and then there's nothing to worry about. But if a blood test shows she's not immune, we'd give her the same protective antibody treatment to prevent or soften the illness. The regular chickenpox vaccine can't be given in pregnancy, which is why we use the antibody instead. None of this is because they're sick now — it's to keep them from getting sick, and especially to protect the baby your wife is carrying." [4] [2]