Vaccines and Antivirals — What Is Safe During Targeted Therapy

Yes — the injected inactivated flu vaccine is generally recommended during targeted therapy, because flu itself can be dangerous when your body is under the additional stress of treatment. Ask specifically for the injected vaccine, not the nasal spray. The nasal spray uses a live attenuated virus and is not safe during active cancer treatment. Your immune response to the vaccine may be lower than usual, but getting vaccinated still provides meaningful protection. ASCO guidance recommends staying up to date with inactivated vaccines during cancer treatment.

The inactivated and mRNA COVID-19 vaccines approved in India are not live vaccines, and they are generally considered safe to receive during targeted therapy. ESMO and ASCO have both recommended that people on active cancer treatment stay current with COVID vaccination. The vaccines do not interact with targeted therapy drugs through the CYP3A4 pathway. Your immune response may be reduced, but the vaccines themselves are not the concern. Tell your oncologist when you are due for a dose so timing can be considered alongside your treatment schedule.

Call your oncology team the same day you test positive, before starting any antiviral. Several COVID antivirals available in India contain ritonavir, which is a very strong CYP3A4 blocker. Taking it without your oncologist knowing can raise your targeted therapy to a significantly higher level than intended, increasing the risk of serious side effects. Antivirals that do not contain ritonavir may be an option and are worth discussing. Your oncologist and the doctor prescribing the COVID antiviral need to agree on a plan together before you start anything.

Acyclovir and valacyclovir are cleared from the body through the kidneys, not through CYP3A4, so they do not interact with targeted therapy drugs through the enzyme pathway. For most people on targeted therapy, a short course for a cold sore or herpes outbreak is manageable — but tell your oncology team before you start, particularly if you have any kidney concerns or if the outbreak is severe or spreading. A painful or spreading herpes outbreak during cancer treatment needs proper medical review, not self-management at home.

Fluconazole is a significant CYP3A4 blocker and it interacts with many targeted therapy drugs. Even a short course for oral thrush can be enough to raise your targeted therapy level meaningfully. Contact your oncology team before taking it, and tell them the dose and duration your dentist is suggesting. There may be an alternative antifungal with a lower interaction risk, or your oncologist may want to review your targeted therapy dose during the course. Tell your team first — not after you have already finished.

Oseltamivir for flu and topical antivirals applied directly to skin are generally lower risk from a drug interaction standpoint. Acyclovir and valacyclovir for herpes infections are also not processed through CYP3A4. That said, lower risk is not the same as safe without checking — your specific targeted therapy drug may have interaction concerns beyond the CYP3A4 pathway, and your oncologist is the right person to confirm this. The safer habit is always to mention it before you start, not after.

Your targeted therapy has a prescribing information document that lists known or expected interactions, and your oncologist or clinical pharmacist can check this against any antiviral being considered. If you are ever in a situation where you need an antiviral urgently and cannot reach your oncologist — in an emergency, for example — tell the treating doctor you are on targeted therapy and ask them to check for interactions before prescribing. Bring your targeted therapy tablet packaging, or a written list of everything you take, to every appointment and every emergency visit.

Household contacts receiving most routine live vaccines — including MMR — are generally not considered a direct risk to someone on targeted therapy, according to ASCO guidance. The main exception is the oral rotavirus vaccine given to infants: nappy changes during the two weeks after the dose carry a small shedding risk, and wearing gloves during that window is usually recommended. Recipients of yellow fever vaccine shed virus briefly, and close contact is best avoided for a few days. Tell your oncology team which vaccines any household member is due to receive, and they will advise on any specific precautions for your situation.