sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread

I saw a few other posts about this and understand what the problem is. However I'm struggling to write the code so it avoids the error:

from database import db_session
from database import init_db

database.py

engine = create_engine('sqlite:///'+full_path, convert_unicode=True)
db_session = scoped_session(sessionmaker(autocommit=False,
                                         autoflush=False,
                                         bind=engine))
Base = declarative_base()
Base.query = db_session.query_property()

def init_db():
    # import all modules here that might define models so that
    # they will be registered properly on the metadata.  Otherwise
    # you will have to import them first before calling init_db()
    import models
    Base.metadata.create_all(bind=engine)

flaskserver.py

@app.route('/process_email')
def process_email():
    init_db()
    email_address = request.args.get('email_address')
    print(email_address)
    email_date =  datetime.now().strftime('%Y-%m-%d')
    rec = db_session.query(EmailCampaign).filter(EmailCampaign.email == email_address).first()
    if rec.email_4 is not None:
        rec.email_4_respond = email_date
    elif rec.email_3 is not None:
        rec.email_3_respond = email_date
    elif rec.email_2 is not None:
        rec.email_2_respond = email_date
    elif rec.email_1 is not None:
        rec.email_1_resonded = email_date

    db_session.commit()
    return('done')

I randomly get

sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 9396 and this is thread id 13360.

as an erorr. I think I have to initialize the connection inside the process_email() function right? I've tried it outside the function but also get the same error.

Thanks.